PUNCH LINE

A 'punch line' is the final part of a joke, usually the word, sentence or exchange of sentences which is intended to be funny and to provoke laughter from listeners.
For instance, in the following well-known joke:
: A man walks into a bar with a duck under his arm.
: The bartender asks: "Hey, where did you find the pig?"
: "It's not a pig, it's a duck," the man answers.
: To which the barman replies: "I was talking to the duck."
"I was talking to the duck" is the punch line: if people laugh at the joke, it's when they hear that part.
Punch lines generally derive their humor from being unexpected. "Punch line" is probably an Americanism, but the "punch" could be related to biting lines delivered by the "Punch" character in Punch and Judy shows. In previous centuries, a joke was sometimes a "bite" or a "hit."
The classic stand-up punch line sound is a sting (erroneously called a rimshot) on drums.