a sexual act deemed normally as unsavory or just plain fucked-up. i.e. foot-fucking, or in the eyes of some, anal sex. An extreme case would be barfing on somebody's chest, adding your semen, and scrambling it (see: Boston Scrambler).
Heinous acts of compassion are common in pornos, schock films, Amsterdam, and liberal arts campuses.
"Dude Jordan fucked his girlfriend in the ass and then came on her and had her lick his shit-covered dick off!"
"Wow, what a heinous act of compassion..."
"Are you ready for a heinous act of compassion"
"Sure, you can fuck me in the ass if you want"
"Uhh... I was thinking I'd ----- and --- midget --- egg beater--- and a huge cock ---- black men ---- 13 dildos ---- a rhinocerous ---- and then a jacuzzi full of pepto bismol --- and then we'd shave a cat ----- fungus --- and then I'd cum."