When someone tries extremely hard to be correct about everything/prove everyone wrong, especially over trivial things that no one cares about. "Being a Jerm" also refers to someone who tells you a pointless fact every chance they get.
Peter: yeah that song came out in the 70's
Jerm: No, it came out in 1978.
Peter: like I said..
Jerm: *googles and pulls up Wikipedia page* see, 1978, you were wrong.
Jim: Dude, I want some celery
Andrew: Did you know that when you eat celery you burn more calories than celery has in it?
Jim: no one cares.. stop being a Jerm, man..