WordNet
gag line
n. the point of a joke or humorous story [syn: punch line, laugh line, tag line]
Wikipedia
GAG line
The GAG line, which as an acronym for Goal-A-Game, was a famous ice hockey line for the New York Rangers in the late 1960s and early 1970s, as these linemates averaged over 1 goal a game while playing together. It consisted of Jean Ratelle at center, Rod Gilbert on right wing and Vic Hadfield (also the Rangers team captain) on the left side.
Usage examples of "gag line".
It's a gag line I probably use too often, but the reaction this time was novel.
This made Mike laugh as if it were meant to be witty, a TV gag line.