a slang term for an asshole. Many think the term gets it's meaning from the warmth of a rectum. In cold situations, one can warm their penis, hands, other body parts, or objects inside another's anus; effectively using it as a physiological 'jacket'.
My feet are frozen! Let me wear your Inuit jacket!
This turkey won't thaw in time for Thanksgiving dinner; I'll put it in my Inuit jacket.