The Word Detective writes that the phrase probably evolved from the American idiom “to talk turkey,” meaning “to speak directly and frankly, without beating around the bush.” An early form of the phrase was “to talk cold turkey,” and using “cold turkey” probably was “…a simple, uncomplicated meal, as a metaphor for simple, unadorned, direct speech.” Talking cold turkey came to mean “give it to me straight; tell me the unvarnished truth. “Cold turkey” came to mean “quit suddenly, with no tapering off…”