What is what does it mean to call someone a tool?

Calling someone a "tool" is generally an insult implying that the person is being used or manipulated by others, often without their own awareness or benefit. It suggests a lack of independent thought or agency, and that they are simply serving someone else's purposes. The term can also imply that the person is obsequious or eager to please, even to the point of being subservient. The severity of the insult can vary depending on context and tone, but it's rarely a compliment.