Command. The opposite of "grow up." It serves as an instruction to the listener to cease activities that are considered by the speaker to be overly refined, mature, or stuffy. Often used by people who wish to continue to employ high school jokes about how "gay" someone is.
Friend 1: I don't find it funny when you call people "gay" as a way to say you don't like them.
Friend 2: Grown down.
Friend 2: Grown down.
by Learned Hand 2 February 2, 2010