Meaning

Display one's emotions openly.

Origin

This phrase may derive from the custom at middle ages jousting matches. Knights are said to have worn the colours of the lady they were supporting, in cloths or ribbons tied to their arms.

The term doesn't date from that period though and is first recorded in Shakespeare's Othello, 1604. In the play, the treacherous Iago's plan was to feign openness and vulnerability in order to appear faithful.