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1.
When something is unbelievably crazy.

It is used instead of nuts when describing a person, place or thing.
EXAMPLE 1:

Person 1: Mate did you see that guy juggling like 10 balls at once on TV last night?

Person 2: Yeah man, it was firenuts!

EXAMPLE 2:

Person 1: Last night, I pulled an all-nighter watching Vampire Diaries

Person 2: You're firenuts.
by elliesmellie1234 November 13, 2010