Someone who is ridiculously dumb, make an ass of themselves, and hasn't got a clue about what's going on around them. However, tools are usually used and kept around for either entertainment purposes (ie. getting them to do something really stupid for laughs) or for other things (ie. transportation).
Phil: Hey, that Dick guy, he's a fuckin' tool! He sings "I'm a Little Tea Pot" when I tell him to.
Bob: Yeah, that's the only reason we keep that tool around.