- to wear one's heart on one's sleeve
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:
Iago:It is sure as you are Roderigo,Were I the Moor, I would not be Iago:In following him, I follow but myself;Heaven is my judge, not I for love and duty,But seeming so, for my peculiar end:For when my outward action doth demonstrateThe native act and figure of my heartIn compliment extern, 'tis not long afterBut I will wear my heart upon my sleeveFor daws to peck at: I am not what I am.