English has two expressions referring to turkeys that seem unrelated. To talk turkey means to get down to serious straighforward talking while cold turkey is a term usually applied to withdrawing directly from an addiction: to kick a drug habit cold turkey or give up smoking cold turkey.
We are not suffering from a lack of explanations. One goes back to colonial times when Indians often brought turkeys to the original settlers on the East Coast. Since the turkey was often the coin of trade, “talking turkey” could have meant getting down to serious negotiations. We have no evidence of that.
Michael Quinion picks upon the Oxford English Dictionary’s claim that the phrase at one time meant “to talk agreeably or affably, to say pleasant things”, suggesting that this sense arose around the Thanksgiving table, where turkey has been the central fare for some time in the US. (The phrase definitely came from the US.) The examples of this meaning that I can find are not convincing and Quinion doesn’t provide any.
I am going to suggest another possibility. The OED claims that one of the meanings of this phrase was at one time “to talk in high-flown language”. This makes sense since turkeys strut, their feathers puffed out and their tail feathers fanned like a peacock’s, when they gobble. The OED found at least one example suggesting this meaning from John Beedle’s Sleigh Ride, Courtship and Marriage, a novel by William McClintock published in 1841: “I was plaguy apt to talk turkey always when I got sociable, if it was only out of politeness.”
While high-flown language is looked on disparagingly in the US, it is usually reserved for serious situations rather than affable ones. “Seriously” seems to be the meaning of the phrase in this line from A Little Bit of Tid-Re-I, II, published in 1824: “So that, all things considered, I hope neither the Indian, whom the Yankey could not cheat in the division of their game (a turkey and a buzzard,)..will accuse me of not talking Turkey to them in this article.”
The 1903 issue of Dialect Notes II has an entry: “Talk turkey, v. phr., to talk plainly: ‘I’m going to talk turkey with him and see if I can’t get him to mend his ways.'” The American preference for straightforwardness in serious discussions could have pulled the meaning of this word in the direction of “plainness” but something else happened at about the same time that makes this even more likely.
Early in 20th century this talking turkey was sometimes extended to talking cold turkey. This expression clearly meant “talking plainly but very seriously”. Cold implies the status of people and machines before they are warmed up, right after starting, and fits the sense of plainly, i.e. without preparation, rehearsal or warming up. The January 4, 1928 issue of The Daily Express (11/5) contained the following sentence: “She talked cold turkey about sex.”
Now, even though the OED’s examples seem to reverse the chronology, I am suggesting talk cold turkey in the sense of “talk plainly and very seriously” became simply talk turkey, not vice versa. We should not depend too heavily on published sources here. Talk cold turkey may well have preceded talk turkey in the sense of “plainly and seriously” in speech but took more time finding its way into print.
The interesting aspect of the extended phrase, talk cold turkey in the sense of “plainness”, is that the other mysterious turkey phrase is cold turkey. Quinion says that this sense is unrelated to the first; but is it?
If cold turkey meant “plainly and very seriously” at the turn of the century, could it not easily have migrated to the sense of kicking a drug habit plainly and very seriously, which is to say, without help, intervention, or preparation? It would have been strongly influenced by the phrase cold sweats, a state which often accompanies withdrawing from an addiction. I think this history makes eminent sense and is supported by evidence, however scant it may be in spots.
A side note: turkey also meant “nothing, not a word, diddledy, squat”, toward the end of the 19th and beginning of the 20th century as we see in this sentence: “You never said turkey to me about leaving.” This meaning no doubt derived directly from the phrase talk turkey. If you talk turkey, turkey is what you talk, i.e. words.