Short for "cisgender" (opposite of "transgender"), used to describe someone whose gender identity matches their anatomical gender at birth.
Occasionally used derogatorily.
Mark: I heard that Stacy decided to start her transition.
David: Huh? Dude, Stacy is cis. You mean her sister.
Occurs when a gay guy does something -- usually while drunk -- that leaves no doubt in anyone's mind regarding his orientation
See: Queen Out
"Damn, Cameron knew all the dance moves to that Beyonce song at the bar last night"
"Yeah, he really dropped his purse, didn't he?"