"Just when you thought it was safe to go back in the water," e.g

• Trailer phrase
• "She walked off the street, into his life and stole his heart," for "Pretty Woman"
• Movie poster words
• "In space no one can hear you scream," for "Alien"
• "One man's struggle to take it easy," for "Ferris Bueller"
• "Good night, and good luck," e.g.
• Genitals mostly involved as point of joke

Wiktionary Word definitions in Wiktionary
n. 1 The punch line of a joke. 2 (context computing English) A pithy quote habitually appended to a signature, used as an advertising slogan, etc. 3 A line attached to a draft of cargo or a container to provide control and minimize pendulation of cargo ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"punchline of a joke," 1926, originally "last line in an actor's speech" (1916), from tag (n.1) + line (n.).