Q: Does the expression “heart on sleeve” come from jousting? In medieval times, a knight would wear a piece of silk the color of his lady’s livery on his sleeve as a sign to spectators of his devotion.
A: The earliest published reference, as far as I can tell, comes from Shakespeare. In Othello, Iago says: “But I will wear my heart upon my sleeve / For daws to peck at: I am not what I am.”
Where Shakespeare got it, heaven only knows. But I think you’re right—the jousting business sounds like the most plausible explanation.
In tournaments in the tilting-yard at Whitehall, the knight designated as the “champion” of Queen Elizabeth I wore the Queen’s glove pinned to the flap of his hat.
According to the 19th-century Dictionary of Phrase and Fable, in medieval times a knight would tie his lady’s “favor” (scarf or ribbon or whatnot) around his arm as a way of symbolically dedicating his performance in the tournament to her. Thus, you might say that just by looking at his sleeve, an observer would know to which lady his “heart” was dedicated.