A poorly defined criticism of another, usually involving projection of one's own insecurities onto that person.
No one actually makes fun of someone because they really are too stupid to know they are being used. This vague usage usually involves the user being jealous of social interaction that is initially caused by someone needing help. Sometimes the "tool" is just happy to help and fully capable of getting anything they want in return, and other times the "tool" may want more but is still getting enough out of it to make the relationship mutually beneficial and incite jealousy.
When used to refer to someone who is "trying too hard" it usually means that the person has a strong influence on social situations in a way that causes jealousy or anger. Again someone who just actually can't think of anything funny to say or just blends in the background isn't likely to be a target.
You might argue that the "tool" may be doing something inappropriate if everyone is mad at the person, but as a general rule intelligent people do not participate in awkward silences when they believe they can offer constructive criticism about someone's behavior.
Gamma 1: God Alpha is such a tool. Doesn't he know Alpha female is just using him for his ride?
Gamma 2: Yeah seriously! Hey shouldn't they be back by now?
Beta M: Hey, I'm just glad I could help. Besides it gives us a chance to get to know each other.
Beta F: Sounds good to me! I caught a ride from those sketchball gammas but they kept acting like I owed them something for it.
After Alpha plus leaves the room....
Beta Minus: God, Alpha is such a tool. Doesn't he know no-one cares about socio-economic theory and how it relates to every day life?
Beta Plus: Sorry what? I was lost in deep profound thought.