What it means:
to display feelings openly
How Shakespeare used it:
Discussing his planned betrayal of Othello, the villain Iago says, "But I will wear my heart upon my sleeve / For daws to peck at: I am not what I am." (Othello, Act 1, Scene 1)
In fact, Iago does not display his feelings for birds to peck at: his true feelings will never be revealed.
"I wear my heart on my sleeve and confessed my true feelings to a man who did not reciprocate. Now I am free to move on ..." Daydreamin on Twitter, Mar. 12, 2010