You've heard of the ugly cry... You cry so hard you look in the mirror to see how bad you look. This is the ugly laugh.
When you laugh so hard you have to look in the mirror just to see the face you are making.
I didn't know my face could make that expression!
It was so funny it made me ugly laugh!
When you are laughing soo hard that you look in the mirror or catch a glance at your reflection and the awkward and ugly face you make. Usually you laugh even harder for the ugly face that you HAVE to make when laughing that hard.
"Oh my God, when I saw that girl pee her pants on the dance floor I couldn't help but to laugh. Then, I realized it was my girlfriend and I started ugly laughing."