1. To warm one's heart.
2. Feeling awe, astonishment, surprise, or admiration.
The way Vicki Valerie sings gives me a major heart-on!
When you really fancy someone and whenever you're around them your heart starts beating faster. Blood flow to the heart, resulting in a heart-on (punned with the more familiar "hard-on"). Sweet!
"You give me a real heart-on girl"