An exclamation used to calm someone down when they are being overly obnoxious or excited about something stupid or insignificant, similar to chill out or relax.
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."
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"