Oxymoron

[lankeela]

Why do we say someone or something is pretty ugly?

And why do we say pretty bad or pretty good, or even pretty much?

[Clanad]

World Wide Words has this to say:
That�s correct. It is first recorded in Old English, when it had the sense of �trick, deceit�. Then it disappears from the recorded language for some centuries, turning up again in the 1400s in a variety of meanings, none of them exactly equivalent to the Old English form. It could mean �clever, artful�, or �something ingeniously or cleverly made�.

Contd.

[Clanad]

Contd. (site still won't take longer posts, apparently)

And it could be applied to a man, as �brave, gallant, warlike�, which weakened down the years until it was used in the eighteenth century in the phrase �a pretty fellow�, meaning a swell or a fop. But the word also existed in a weakened sense, very much like our modern nice�pleasing or satisfactory in a vague sort of way. In this sense it was applied, in rather a condescending way, to young women as a reduced version of beautiful.

ObvIously, as the word aged the meaning became useful in producing oxymorons such as yours...

[aristotle]

These days "pretty" in this context means much the same as very only a bit less so. By the way, only the first one (pretty ugly) is an oxymoron - pretty good and pretty bad are not.