What keeps you warm on the way home from the pub, even though its below freezing and you are in fact wearing only a T-shirt.
As worn, in particular, by inhabitants of northern English towns while on a night out in winter. They don't want to carry their coats around bars and clubs all night, so they go without.
It's freezing out there, and the last bus has gone. Luckily I have my beer overcoat to keep me warm on the walk home.