1.Sugar knuckles is a synonym for oh shit, well damn, fuck, oh crap, and things like that.
2.Where as sugar knuckle is an insult. Sugar knuckle means idiot, douche, asshole, etc.
1.SUGAR KNUCKLES!!! I just ran over an old lady!
2.You're such a fucking sugar knuckle! How the hell do you run over an old lady?!