1.) A guy with a hugely over-inflated ego, who in an attempt to get un-due attention for himself, will act like a jackass, because, in his deluded state, he will think it's going to make him look cool, or make others want to be like him. The person may even insincerely apologize later on, but only in an attempt to get more attention, or to excuse his blatantly intentional, and unrepentant tool-ish behavior.
2.) Someone whose ego FAR exceeds his talent, intelligence, and likability. But, of course, he is clueless regarding that fact. He erroneously thinks he is THE MAN!
3.) Someone who others normally refer to as a prick, dick, or schmuck.
4.) Someone who acts like a dick, because...well...he's compensating.
5.) One who lacks the mental capacity to know he is being used. A fool. A cretin. Characterized by low intelligence and/or self-esteem.
That tool doesn't even know she's just using him.
6.) A person, typically male, who says or does things that cause you to give them a 'what-are-you-even-doing-here' look. The 'what-are-you-even-doing-here' look is classified by a glare in the tool's direction and is usually accompanied by muttering of how big of a tool they are. The tool is usually someone who is unwelcome but no one has the balls to tell them to get lost. The tool is always making comments that are out-of-place, out-of-line or just plain stupid. The tool is always trying too hard to fit in, and because of this, never will. However, the tool is useful because you can use them for things; money, rides, etc.
"Let's drive to Chicago from Iowa. Oh man, we need money. Hey, let's ask tool to come, he'll spot us the cash!"
"Tool is our friend. We call him that because he is one."
7.) someone who is a complete idiot/ one who is used by other people, and usually dosen't even realize it/ someone who can't think for themselves/ an asshat.
8.) A sad soul who is being taken advantage of without realizing it
-Dude, I need a ride to the party.
-Ask (insert name here). He's a tool.