What it means
Means you show your feelings plain as day instead of playing it cool. If you're buzzing, raging, heartbroken, whatever it is, it shows straight away and everyone clocks it. It's usually said with a bit of warmth about someone who's open, honest, and absolutely useless at hiding what they're feeling.
Usage examples
"Don’t ask Jess about the breakup, mate, she wears her heart on her sleeve. Five minutes in and she’s tearing up in the pub."
"He proposed at the station with everyone watching, crying his eyes out. Classic him, wears his heart on his sleeve."
"I can never bluff at cards, I wear my heart on my sleeve, you can read my whole hand straight off my face."
"You can tell Sam's wearing his heart on his sleeve tonight, one tiny comment and his whole face drops."
"She's lovely but she wears her heart on her sleeve, so if she's annoyed with you, trust me, you'll know by tea time."
Where it comes from
This one goes back to Shakespeare's Othello. Iago says he'd wear his heart upon his sleeve for daws to peck at, meaning showing your real feelings leaves you exposed. English kept that image, then softened it over time into the modern line for someone who's emotionally open and easy to read.
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