A perjorative term for someone so ugly, so exceedingly unattractive. that he or she is as "ugly as a butt" (a misnomer, since many buttocks are beautiful indeed). A popular teen-age slang term since the 80s, "butt-ugly" most probably began as military slang c. World War II. Probably because enlisted men found nothing attractive about another enlisted man's hairy, sweaty, very possibly pimpled butt!
That dude is so butt-ugly that just the sight of him makes me wanna puke!
A condition of being extremely ugly.
That dog is butt-ugly.