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1.
The feeling you get instantly after ejaculation when you have had sex with someone proper rancid due to slcohol. The pure natural and unstoppable instinct of emptying your sack combined with copious pints of ale takes over all rational thought then instantly returns the split second you orgasm

similar to instaguilt
Dave: - "mate i pulled this munter last light with my beer goggles on... but fuck me i got instaregret the moment i blew my load"

Tony:- "lol, i know what you mean... its like the worst feeling ever"
by mordzy2 October 18, 2010
 
2.
Regretting something almost immediately after it happens. Usually leaves you wondering why you did what you just did.

Often occurs after text messages, sent e-mails, and that last drink you shouldn't have had.
"You probably shouldn't drink any more of that tequila, I bet you'll insta-regret it."

"Shut up, don't tell me what to do! (takes a swig) ...Ahh crap, I feel nauseous."
by The Luke Man October 10, 2009