When a person laughs excessively hard at something that is either mildly funny, not THAT funny, or just plan not funny at all. The person displaying the overlaughter is easily amused and doesn't actually get the joke. In other cases, lame people are just nervous while being around cool people they're interested in dating and so they overlaugh to compensate for a lull in conversation, which usually backfires.
Girl: Dude and I were just watching tv, and this commercial came on. It made me smile, but he completely overlaughed at it. I mean he was still laughing when the show came back on.
Homegirl: For real, girl? Dude sounds lame. Are you sure you want to go through with this?
Girl: I don't know. He may just be nervous, but I'm going to need him to tap into his inner cool.