Somebody who is a complete retard, normaly said when someone ither does something stupid or just to put them down if u think it wud be funny. It originates from people who are too dumb to go get a knife so instead jab at there food with a spoon. Basically a similar word would be retard.
Guy 1: Yo phil your a fuckin spoonjab man
Guy 2: Shut up gaylord ur a spoonjab