1. To crack a huge grin
2. When something makes you so happy that it makes you smile.
3. To make someone else smile
This term originates from the word cheesin, which means having a big smile on your face.
1. I brought something in my bag that's gonna cheese you up!
2. I was so cheesed up when I got accepted to the uni of my dreams
3. Dawg what's wrong with you? You've been frowning all day. Cheese up!