The invisible coat you where over your tshirt/short sleeve/bare chest when walking home on a freezing night out without feeling the cold.

"Aren't you cold?"

"No i'm wearing my beer jacket!"

"Nice one"

The ability not to feel the cold when drunk. Also know as a beer coat

"You must have been freezing last night walking home?"
"I didn't feel it. I had my trusty beer jacket on"