This phrase may derive from the custom at middle ages jousting matches. Knights are said to have worn the colours of the lady they were supporting, in cloths or ribbons tied to their arms.
The term doesn't date from that period though, and is first recorded in Shakespeare's Othello, 1604. In the play, the treacherous Iago's plan was to feign openness and vulnerability in order to appear faithful:
It is sure as you are Roderigo,
Were I the Moor, I would not be Iago:
In following him, I follow but myself;
Heaven is my judge, not I for love and duty,
But seeming so, for my peculiar end:
For when my outward action doth demonstrate
The native act and figure of my heart
In compliment extern, 'tis not long after
But I will wear my heart upon my sleeve
For daws to peck at: I am not what I am.
- To express an emotion, belief, or stance overtly and make it an important part of one's public life.
- But religion is the one thing they encourage you to wear on your sleeve. (New York Times)
- While he may wear his idealism and good-heartedness on his shirt sleeve, his passion is exuded more as an intellectualized than as a deeply felt quality. In modulating his voice, that is, Canin has chosen a character whose temperance might be admirable among the living but is less engaging when encountered in literature. (Chicago Tribune)