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.
The influence that the E4 TV Show 'Skins' has on teenagers, meaning that they want to be like their favourite characters, doing things just because they see it, and think it's cool.
'Hey, want to want to go the woods and do drugs?'
'Erm, I think you're a little under the skinfluence'