An adjective used to describe something that is simultaneously fantastic and evil or terrible, such as drinking a beer someone was saving for themselves, driving someone else's nice car without permission, or sleeping with someone who's cheating on their boyfriend/girlfriend. In other words, a good thing made better by it being forbidden, but which also causes a bit of guilt.
I promised myself I'd stick to my diet, but I ended up eating some of that cheesecake anyway, and it was sinfulicious.
Man, that married chick is so hot, it's sinfulicious to be watching her like this.
I stole these sunglasses from my brother. They're awesome, but he's pissed. Totally sinfulicious.