When you do a crazy move while playing keyboards, but not too crazy. It's when you're wild, but still mantain composure.
It originated from Jesse Johnson, moog player in MCS. He does wild moves all the time, so a "half-jesse" is half of what he would do at a show.
Bill: Dang, that keyboard player is up there doing some sweet moves, yet still manages to maintain a professional feeling about him.
Jim: I guess you could say he was doing a half-jesse!