slang: term used to describe something that was really good or exciting
Aaron: yo that movie was real good.
Sandy: yeah. that shit was fire.
may also refer to particularly good weed.
also a metaphor used as a substitution to "hot" as in appealing in appearance. "on" precedes it on most occasions.
2. This shit is FIRE! i can't even feel my own appendages!
3. That bitch is on fire!