It’s an idiom in English (in American English at least, I can’t speak for the other dialects). “Nut” can be slang for a crazy person, e.g. “That guy’s a real nut.” Other variations are “nutters” or “nut case”. A “<topic> nut” is someone who is “crazy” about that topic.
What does it mean? Someone who’s really into sports??
Yes
It’s an idiom in English (in American English at least, I can’t speak for the other dialects). “Nut” can be slang for a crazy person, e.g. “That guy’s a real nut.” Other variations are “nutters” or “nut case”. A “<topic> nut” is someone who is “crazy” about that topic.