2) refers to a boyfriend who has weird habits, crab claws your chest or wont have sex with you even though he has no chance of getting laid.
3) someone who has bad acne and sort of looks like a gremlin because of it
2) Mcnasty is coming over but we wont DO anything.
3) I got put on this cream because my face was mcnasty.
Bro 2: Lets go to Micky D's
Bro 1: Im totally getting a Mcnasty!!
Bro 2: Dude you're sick!