A valediction which has come to mean "be cool," but which more properly understood is an admonishment to stay alert and on one's toes. See the quote below for the pop culture origin.
"Hey! I know we're all in strung out shape but stay frosty and alert. We can't afford to let one of those bastards in here."
--Corporal Hicks (Michael Biehn), in Aliens (1986)