It's a more sophisticated way of saying "alcohol jacket." Saying that you're wearing an alcohol jacket makes you seem a little less classy. Not only is saying you're wearing a hypothetical coat more entertaining, it's also a little less obvious.
After a night of drinking:
Guy 1: "Man! It's so cold out here. I can't believe we're drunk and walking home in the snow!"
Guy 2: "I know!"
Guy 1: 'Hey, aren't you cold in just a t-shirt?"
Guy 2: "No way man! I'm wearing my hypothetical coat!"