Originated from the motion of sliding the back of your hand up your throat and doing a nice little flick at the end of the whole motion. Also known as fuck you.
Man: "Hey baby, wanna come over my house and bang?"
Woman: "Chin up!"
Metaphor to keep one's head up and eyes forward on what lies ahead, opposed to how typically one's gaze tends downward to the ground while in a sullen state.