In theory, it refers to someone who believes everything is a conspiracy. In practice, it is an overused ad hominem tactic against someone who believes that some incidents could be a conspiracy. True conspiracy nuts are pompous teens who want to feel important, but people who generally use the term are on the opposite end of the spectrum and afraid to acknowledge the possibility of a reality different from their perception.
Sensible thinker: All I'm saying is that there could be more here than meets the eye.
Tool: ur just a conspiracy nut