utterly disgusting; highly sickening.
Ex.#1:person #1: "Amy Whinehouse's teeth and hair are butt nasty."
person #2: "I know!"
Ex.#2:girl #1: OMG, last night during the slumber party, my BFFs,like, dared me to,like,wear mascara on my lips and wear leg warmers in public!
sister: OMG, ewwww, that is soooooo butt nasty!
Ex.#3: tween boy #1: did you see the picture on page 52 in our health book? you know, the detailed pic about...
tween boy #2: yeah, that was butt nasty...
ex. #4: ghetto guy #1: man, i was at this one pool party yesterday, and when i went into the pool i saw my girlfriend with a booger danglin' from her nostril!
ghetto guy #2: Man, dat is butt nasty!