Cold turkey is an expression used when someone gives up some sort of addiction - frequently in relation to drugs. We say they are going cold turkey meaning that they have stopped taking whatever they are addicted to, without help from professionals and without taking any substitute for their addictive substance.

"To go cold turkey" means to suddenly give up an addiction. It originally referred to quitting heroin, but it's used in a general sense now. One theory behind the expression comes from the cold sweats and the goose bumps that accompany drug withdrawal symptoms... like a cold turkey.