The phrase to WEAR YOUR HEART ON YOUR SLEEVE means you are displaying your emotions to everyone.
This phrase could have derived 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. However, it was found before that period. It 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.