Sexually desirable. Usually used in reference to a female, and usually specifically used for a woman with a full and well rounded butt.
Originally, it ment that something tastes (or looks, or is) like ass, which is to say, bad.
Yes, bootylicious is a contronym.
Etymology (could we be any more uncool?)
Well, it's a contraction of booty and delicious. 'Booty' is a slang word probably derived from the English word 'body', but which has come to refer almost exclusively to rear ends. 'Delicious' comes to us straight from the Latin 'delicia', meaning 'pleasure'.
We don't know who first used the word, but the first reference to Bootylicious in popular culture is probably by Snoop "Doggy" Dogg in the song Dre Day on Dr. Dre's 1992 album The Chronic.
Your bark was loud, but your bite wasn't vicious
And them rhymes you were kickin' were quite bootylicious
The word was later used, in the currently more popular, positive sense in a song called Bootylicious, written by Beyoncé Knowles for Destiny's Child in 2001.
I don't think you're ready for this jelly
I don't think you're ready for this jelly
I don't think you're ready for this
'cause my body's too bootylicious for ya babe
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