The act performed when "packing" a pack of cigarettes. Typically the smoker will utilize the thumb and forefinger of the dominant hand, pinching the pack of cigarettes with the top of the pack facing down. The pack is then smacked against the palm of the opposite hand.
Joe, "Don't you think those smokes are dead?"
Man, "I'm clapping for cancer."
Joe, "ah. The good 'old Cancer Clap."