Where did the phrase “to wear one’s heart on one’s sleeve” come from?

Though Shakespeare was the first to use the saying “to wear one’s heart on one’s sleeve”, thus indicating an ostentatious display of one’s limitless devotion, he was merely adapting another phrase current in his day and which he himself used in an earlier play.

It was lago, in Othello, who wore his heart on his sleeve, professing a devotion to his master, Othello, which, with him, was altogether feigned.

The usual phrase of that period was to pin (a thing) upon one’s sleeve.

Shakespeare uses this in Love’s Labor’s Lost, where Biron, speaking of Boyet, says, “This gallant pins the wenches on his sleeve,” meaning that Boyet is openly devoted to all wenches.