Extremely disgusting, or filthy; excessively nasty; nasty(X10).
"Dude, did you have to blow up my bathroom? It smells MAGNASTY in there."
"Hey look at that chic. Does she have armpit hair? . . . damn, that's MAGNASTY."
"Why the hell do you have beastiality porn on your computer!!! You're fucking MAGNASTY."