A vile and morally deprived individual. Usually suffering from a number of mental afflictions that are not limited to but can include, necrophelia, beastiality, compulsive masturbation, voyeurism, exhibitionism, and compulsive addiction to pornography. The only reason that a "Johnny McNasty" cannot be classified as a sexual predator is because they only prey on themselves. Generally a "Johnny McNasty" can be identified as anyone who has the tendency to place his or her genetalia on anything that is made up of atomic matter, and everything in the universe is made up of atomic matter.
ex. I caught my cousin fucking a fresh (still warm) dead chicken while wearing a gag ball and watching midget clown porn and fingering himself in the ass and screaming "please oh please don't stop Santa!" at the top of his lungs in the middle of a laundry mat. He is such a Johnny McNasty.