An exclamation used to calm someone down when they are being overly obnoxious or excited about something stupid or insignificant, similar to chill out
Usage has nothing to do with whether the object of the phrase actually has tits.
Sam-"Jimmy, come ONN, give me a piece of Wrigley's supermint."
Jimmy-"Chill your tits."
Ninth Grader-"Oh my god, those girls are sooo hot, i like the one in blue. Do you think i should go talk to her?"
Eleventh Grader-"Chill your tits, kid, it's just a sixth grade birthday party."