An Anorak is a term from British railfandom which has seen some usage among ALEs and refers to someone so intensely obsessed with detail as to be not very fun to be with. Mildly derogatory. Compare Rivet counter and see wikipedia:Anorak (slang).
The first Anoraks were British fans of things. The derivation many are familiar with is for railfans, who would hang out at stations and record the makeup of trains (how many cars, what car number each one was, etc) and would get to the point where they could predict what cars would be on what trains in what order during the day. They (apocryphally) wore large coats (also called anoraks) in winter and the name transferred over.