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

• Identifying phrase
• "She walked off the street, into his life and stole his heart," for "Pretty Woman"
• Catchword; slogan
• Movie poster words
• Something funny in setting of gelatin
• "Good night, and good luck," e.g.
• Entertainer's signature

Wikipedia Word definitions in Wikipedia
In entertainment , a tagline (alternatively spelled tag line ) is a short text which serves to clarify a thought for, or is designed with a form of, dramatic effect . Many tagline slogans are reiterated phrases associated with an individual , social group ...

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.).