When combined with "ringer" (which is slang for "counterfeit"), we have a phrase meaning "perfect imitation" or "absolute duplicate." Someone who is a "dead ringer" for another person is one who bears so much of a resemblance to the other that they could be mistaken for twins (Hendrickson, pp. 573-574). However, there is another possible origin for this phrase, suggested by "The Word Detective," Evan Morris. He asserts that the "ringer" in this case originates with the phrase "ring the changes," meaning "to ring all the bells in a bell-tower in varying sequences," and then repeating that in various ways. "Ring the changes," then, is slang for substituting something negative (wrong or bad) for something positive. It is a poor substitute for the real thing. Supposedly originating about 1890, it began as slang or argot (specialized communication) of the horse track where a less-qualified horse was substituted for the better animal. Still used to refer to tampering with or substituting for the agreed upon superior entity, a "dead ringer" is something to avoid (Morris).
Few would argue with the concept that when one is dead, that is absolute (one cannot be "somewhat dead" or "a little dead"). When a ship is "dead in the water," it is not able to progress by any means (sail, power, or hand). That phrase, taken from nautical terminology, has come to mean any issue that is absolutely finished or no longer making progress (Scorpio Tales).
Along those same lines of meaning "absolute," we have the phrases "dead run" and "dead heat." The former means to go as fast as one (most often a horse) possibly can. Some believe that a horse will run, if provoked to do so, until it collapses and dies from the exertion (Drake, p. 85). The latter term refers to the horse race (or other competition) that ends in a tie (a "dead heat"). In trotting races, each time around the course is called a "heat" and the object was to win two out of three or three out of five trials or heats. When horses tied in the heat, it did not count towards a victory; hence it was "dead." Now the phrase has been adapted to any tie, and the "dead" qualifier has come again to mean "absolute" (Castle, p. 71). Like the "dead heat," which ends with no winner, a "deadlock" is when there is no further progress in the endeavor. Its first known usage was in "The Critic," by Sheridan in 1779 (Harper).
The number of terms in which "dead" is used to mean "unwavering," "correct," or "absolute" is staggering; and they come from a number of different fields. We find this synonym for "dead" in such phrases as "dead tired," "dead set against," "sick to death" (Magnuson), "dead ahead," "dead on," "dead straight," "dead certain," "dead center," "dead right/wrong," "dead broke," "dead asleep," "dead end," "dead stop," "dead shot" (not to be confused with "shot dead," though being a dead shot would help in accomplishing that action), and "dead silence/calm." In the UK, the term "dead" as synonymous with "complete" dates back to the 1500s; the concept of being dead was so absolute and complete that it left no other option (Morris). Ironically, this was also a time when a person might be pronounced dead when, indeed, he/she was merely in so deep a coma that the body appeared lifeless to all available means of detection (Balmer).
This use of dead as meaning "absolute" is probably part of the expression "dead to rights." The phrase dates back at least to the mid-1800s as a slang, or argot, but its parts, as already partially discussed, date back much further. "To rights," in the 1300s, meant "in proper manner," eventually evolving to mean "in proper condition or order." We can see this usage in the phrase "to set to rights," which means that we are making the situation as correct or orderly as possible (in usage by the 1660s). So when a criminal is caught "dead to rights," he is most completely apprehended with all the paperwork and/or evidence in proper order for his conviction (Morris). (Note: while some might argue that "dead to rights" is a reference to the Catholic ritual of giving last rites, one need only check the spellings of the two words, "rights" and "rites" - short for "ritual," to realize that such an origin is not accurate.)
Then one might believe that "loving a person to death" means loving that person completely. Perhaps. Or, possibly, it could refer to the concept of death being the ultimate conquest - in Medieval times, a knight might "capture" his intended bride, such capture being the death of her freedom (and her virginity) (Sorenson). (In the ideal situation, of course, the maiden would also "love him so much she could die," making her change of status to matron much more enjoyable.) Of course, today we don't look at loving a person to death as meaning or intending the literal demise of that person - that would make such a proclamation rather morbid - but we may be expressing an exclusivity of our love with that remark (and hoping that the emotion is mutual).
Something or someone that is useless (completely worthless) is often described as being "dead wood" (Carter, p. 130). It would seem to me that just saying a person was "wood" would clarify that he/she had little value as a person; does "dead wood" make that individual even more valueless? In 1500s England, the common people were permitted to gather, for personal fires, branches that had fallen from and/or any dead wood still on the tree (the forests being the property of the king, of course), as long as it could be reached with a shepherd's crook. This wood was presumably useless to anyone of a higher position in the community and thus was fit for those of the lower classes (Crystal). Considering this phenomenon, it seems to follow that any useless item could similarly be classified as "dead wood" (but don't try to tell a peasant that his warm fire from the newly gathered fuel is created from something useless).
The theatrical expression "knock 'em dead" has slowly moved into other arenas (including the job interview, where, at least in most recent times, this could have a sinister meaning if a person failed the interview but proceeded in following the advice). One guess is that the phrase is a variation on "killing them" (meaning giving the audience such a great show that they, perhaps, "die laughing"). One hopes that one's performance is so amazing that the audience will actually remember it, not die before leaving the theater, but the imagery of the attendees being "mowed down" by the intensity of the performer is something that presenters strive for (Berg). Again, this may well be an example of the thoroughness of the actor/singer/dancer to entertain his/her audience to their complete satisfaction with the performance.
Of course, for genealogists, one meaning of death is the opportunity to get information on our ancestors. We take off at a dead run for the library, hoping for success. We are not looking for just a dead ringer for our forebear, we want the genuine article and will search to the death for every clue. As we pour over books in the library, there is dead silence as we hope we are dead on the trail of, well, the dead. We may be dead tired by day's end, but if we are dead certain we are dead right, it is OK that we might end up dead broke from the copying costs: we found our ancestors, dead to rights, and we love them to death. If, however, we come to a dead stop, being dead in the water because we hit a dead end, we discard the deadwood (books, films, and websites that turned up nothing in our favor), go home and fall dead asleep. Tomorrow's another day and, while others might be sick to death of this relentless searching, we go dead ahead back into it, looking to break the deadlock. Unlike death, it is an endless quest.
References
Balmer, Emma. "Buried Alive! (Premature Burial, Urban Legends, and Preventative Measures)." Hubpages. Accessed August 12, 2009, from http://hubpages.com/hub/Buried-Alive-.
Berg, R. "Re: Knock 'em Dead." The Phrase Finder. April 12, 2001. Accessed August 12, 2009, from http://www.phrases.org.uk/bulletin_board/8/messages/552.html.
Carter, Ronald. Language and Creativity: The Art of Common Talk. London: Routledge, 2004.
Castle. Why Do We Say It? The Stories behind the Words, Expressions, and Cliches We Use. Secaucus, NJ: Book Sales, Inc., 1985.
Crystal, David. "What's so Special about Bricklehampton?" The Guardian, May 19, 2007. Accessed August 12, 2009, from http://www.guardian.co.uk/books/2007/may/19/featuresreviews.guardianreview3.
Drake, Paul. What Did They Mean by That? A Dictionary of Historical and Genealogical Terms Old and New. Bowie, MD: Heritage Books, Inc., 2003.
Harper, Douglas. Online Etymological Dictionary, 2001. Accessed August 12, 2009, from http://www.etymonline.com/abbr.php.
Hendrickson, Robert. The Facts on File Encyclopedia of Word and Phrase Origins, Rev. & Expanded Ed. New York: Checkmark Books, 2000.
Morris, Evan. The Word Detective. Accessed August 12, 2009, from http://www.word-detective.com.
Scorpio Tales. Diversions. Accessed August 3, 2009, from http://users.tinyonline.co.uk/gswithenbank/sayindex.htm.
Sorenson, True. "Death & Co. by Sylvia Plath." Experts: Literature. January 7, 2009. Accessed August 12, 2009, from http://en.allexperts.com/q/Literature-697/2009/1/Death-Co-Sylvia-Plath-1.htm.