Wear your Heart on your Sleeve

Meaning:
To display how one is feeling for all to see.

Example:
Richard is someone who wears his heart on his sleeve. He couldn’t hide his disappointment when he lost the competition.

Did you know?
This saying might have originated from jousting matches in the Middle Ages. Back then, knights apparently tied ribbons or cloth strips to their arms to show which lady they were supporting.

Reference: http://www.phrases.org.uk/meanings/403000.html