This is an idiom that has been around for several centuries. When you wear your heart on your sleeve, you make your feelings clear to everyone; in other words, you don’t hide your emotions. If you are in love with someone, you make it known to everyone that you are crazy about this person.
*It’s difficult to know what Asha is thinking. She doesn’t wear her heart on her sleeve.
During the Middle Ages, when knights fought each other, they would dedicate their performance to a woman of the court — usually someone they were in love with or cared for. To let everyone know who they were in love with, the knights used to pin onto their sleeves a hanky or a scarf belonging to the woman.