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1.
A cross between the phrase "butt-ugly" and the word "nasty." In other words, it is the highest degree of nasty.
"That...is the most BUTT-NASTY FACE I have EVER seen."
by Darian Johnson May 15, 2006
 
2.
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!
by codenamejester May 16, 2009