What does "wear your heart on your sleeve" mean — and why is it funny?

informal

Meaning

To show your emotions and feelings openly, where everyone can see them.

Where it comes from

An old phrase, found in Shakespeare's Othello. It may nod to a custom of knights wearing a lady's token on their sleeve at tournaments.

Why it is funny

The mild humor is the literal, slightly alarming picture. Your heart, the most private organ you have, is pinned to your sleeve like a brooch for the whole world to inspect.

Used in a sentence

"You always know how he feels — he wears his heart on his sleeve."