like 'oh crap!'. its like when you swear when you do something wrong or hurt your sorry lil ass. i think it originates from that angry kid tv show. Ross Fowler says it a hella lot.
Henry Fielding: Word has it you're a buggeranto?
William Flynn: Never, yesterday I was swiving a whore!
Henry Fielding: No sir, I put it to you that you are a molly fuckster and were seen pulling another man's goose?
Similar to the fart version, the "buttercup" , a "buttercunt" is when a girl quiffs into her cupped hand and masks it over another's face to be smelled.