Green’s Dictionary of Slang

cold turkey adv.

[cold turkey n. (1)]

1. [1910s+] (US) directly, openly, candidly, without any warning.

2. [1920s–50s] (US) caught in the act, obviously guilty, without any excuse.

3. [1950s+] (drugs) of withdrawal, without any form of medication to modify the pains.

4. [1960s] without any alcohol or narcotics, although not intending withdrawal.

5. [1990s+] in fig. use, denying oneself.

In phrases

talk cold turkey (v.) [ext. of SE colloq. phr. talk turkey]

[1910s+] (orig. US) to speak frankly and without reserve, to talk hard facts, to get down to business.