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!