Orphaned punchline

When a scene (film/tv show etc) starts with someone telling the punchline of a joke, without the setup. Usually followed by loud, overly-dramatic laughter from those present.

(Name taken from Tv Tropes).

“So he says “do you love me?” and she says “no, but that's a real nice ski mask!” - Dumb and Dumber.

Orphaned punchline