When a person wears one pair of underwear for several days, and then flips the same pair of underwear inside-out to wear again for another few days.
The act is usually performed by those who practice poor hygiene and selective laziness. The common end result is a thick and potent load of fecal matter buildup inside the underwear between the anus and genitalia.
The accumulated fecal buildup can become black and encrusted over a matter of hours or days. One who practices the Double Fudge Brownie is often at risk of social rejection, and/or public humiliation.
"We tried to have sex but when we got naked and threw our undies on the floor, I saw the inside of hers was smeared with a double fudge brownie which made my boner go down quick!"
"I was gonna make my cell mates butt hole wider but when I stuck it in the first time he was sporting a double fudge brownie, so I stabbed his ass and went to bed."