A tool is many things it can be used to build stuff.
It can be a person who says the wrong shit.
It can be an anonym for cool.
The most common explanation is that it's referred to a poser.
However most of the labelers of this word commonly have a spec in their eye when they label tools. Most people throw the term around so much without realizing that they're an idiot. Conicedently, another defination for tool is someone who doesn't realize they're an idiot. Which means, that most people who use the word tool are tools themselves. Proabably most people (not me) who post on this site about the urban defination of the word tool are coincidently tools.
(Sees a guy wearing a shirt with a message) Hey why the fuck are you wearing that shirt you fucking tool! Poser!(Doesn't realize that the shirt is portraying a completely different message).