Comes from the spanish word, "Nalga" which is slang for butt. When you're making out with a fine ass lady and you take off her pants and her ass is full of pimples, or celulite, looks like cottage cheese, or any other kind of unpleasant surprise.
I was about to stick it to this hoe Cindy, when she liffted up her skirt and BANG!!!! Nasty Nalga Surprise.
Dude that sucks..so did you bang her?