Somebody who believes that they can multitask, but in attempting many different endeavors, fails at all or most of them. A failed polymath. An incompetent, wannabe rennaissance man.
"Dude thinks he's the jack of all trades. He screwed up the math on the cash register, he can't speak spanish for shit, and after he tried to tune up my car, I had to take it in and spend $200 bucks before it would start again. He's the jackoff of all trades."
A graduate of a high school or college, usually male, who still hangs around his former school. This is usually because said individual is a total loser, is trying to meet younger girls, and has nothing better to do.
"Dude, didn't that guy graduate like two years ago"
"Yeah, but he still hangs around here trying to mac it to freshman girls. He's a total alumnumnuts."