To quit something all at once, with zero tapering, no patches, no "just one more", nothing. Usually used for addictions like smoking, booze, sugar, or doomscrolling. It implies you are bracing for the nasty bit, cravings, mood swings, the lot. Brutal, dramatic, and weirdly admirable if you actually pull it off.
"I quit vaping cold turkey and now I’m pacing the apartment like a raccoon that lost its snacks. My mate offered gum and I nearly cried in aisle three at Target."