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
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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
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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
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