The definition for when 2 guys have had intercourse the same girl.
Derived from a show called "The League". This was introduced by actor/comedian Jon Lajoie
"Dude, I had sex with Cindy last night!"
"Oh yeah... that's cool. I fucked her 2 weeks ago. That means we're Eskimo Brothers"
"What? We've been going out for months..."
"Hey bartender, let me get a free drink. We're eskimo brothers!"