To be a "tool" in slang terms generally means to be a foolish or easily manipulated person who may lack awareness of being used by others. It often carries negative connotations, describing someone who tries too hard to gain social validation or appears self-absorbed, egotistical, or lacking in intelligence or self-esteem. The term is sometimes used interchangeably with insults like "idiot" or worse vulgar terms related to male anatomy. A "tool" might also be someone who strictly follows rules to the point of being a nuisance or who excessively cares about appearing cool or fitting in but ends up being mocked by peers.

In its original, non-slang sense, a "tool" is any physical device or instrument used to perform a task or make work easier, such as a screwdriver or a gardening tool.