Radio comedian Fred Allen once said that a good joke should have the same impact as a punch in the belly.
The “punchline” is the twist that makes a joke funny, and the term was in use long before Fred Allen.
It first appeared in Variety in 1921, but its use as the end of a skit goes back to the medieval husband and wife puppets Punch and Judy.
Each skit ended with Punch getting the best of Judy, which gave us the expression “pleased as Punch.”
A good punchline makes a comedian “pleased as punch”.