From today's W.A.D.:

This week's guest wordsmith, Dr. Mardy Grothe (drmgrothe@aol.com), writes:

...
In an oxymoron (like pretty ugly, old news, or sweet sorrow), words that are incompatible or that contradict one another are joined together in a way that ends up making a lot of sense....

This got me to thinking: how have we come to use pretty as an adverb in this sense?