(n.) The state or condition of one who has had sex with one's own grandmother in the 1950s, thus resulting in being one's own grandfather in the present/future. This condition may create genetic abnormalities.
Nibblonian: "It's a genetic abnormality which resulted when you went back in time and performed certain actions which made you your own grandfather."
Fry: "I did do the nasty in the pasty!"
Nibbler: "Verily. And that past nastification is what shields you from the brains. You are the last hope of the universe."
Fry: "So, I really am important? How I feel when I'm drunk is correct?"
Nibblonian 1: "Yes. Except the Dave Matthews Band doesn't rock."
Sexual intercourse that happend in the past.
Dude 1: Dude, I think your past nastification may have some repercutions.
Dude 2: I did do the nasty in the pasty.