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"
by justpaul February 23, 2005
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"
by !david clarke! May 29, 2007
the lack of a need for a real jacket due to the warming effects of alcohol
dude are u not cold?? no ive got a beer jacket on!!
by Rossa O' Donovan January 22, 2009