Anyone who tries to act cooler than they actually are. Most tools are usually douchebags with negative self-esteem.

Synonyms: Asshole, poser, faker, douchebag, Dane Cook.
Man: Wow, Dane Cook is not funny at all. What an asshole.
Man 2: Damn, you're right. What a tool.
by BigCron August 12, 2009
Tool: (noun)

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 unrepentantly tool-ish behavior.

2.) Someone whose ego FAR exceeds his talent, intelligence, and likeability. 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.) Kanye West
Example:

Observer #1: Hey, did you see Kanye West's idiotic stunt at the awards show?

Observer #2: Oh, you mean, "Mr. It's-All-About Me?"

Observer #1: Yeah...what an asshole.

Observer #2: I know. He's such a tool!
by Hilo, Hawai'i September 18, 2009
One who lacks the mental capacity to know he is being used. A fool. A cretin. Characterized by low intelligence and/or self-steem.
That tool dosen't even know she's just using him.
by tt February 19, 2003
That guy who makes us shake our head in disbelief but at the same time makes us feel better about ourselves since we are not him.
"Jake just spent $80 buying those hot chick shots and then they walked off with those other guys!" "He's such a tool!"

by toolguru October 23, 2007
The act of getting used. Unintentionally doing something that someone else gets credit for.
Igor just stole your idea and won the contest with it! Boy, you got tooled!
by T-Rav January 22, 2005
(n.) - an individual who willingly changes his or her behavior for personal gain or profit. In this context, a tool is similar to a poser, with one important distinction. Because a tool does not consider his actions beyond rudimentary cost/benefit analysis, he or she will often be co-opted by corporations or powerful interest groups to support the lame or corrupt positions held by these organizations. A tool therefore, is quite literally an instrument of the man.
by Kenny Genius February 9, 2011
An often disliked person who screws people over instead of using common sense to deal with situations. Likes to command people to compensate for his small penis.
Wtf, Jerry really had to fuck us over this time. What a tool.
by Some dude1231 May 1, 2011