How odd that pretty as an adjective means “having beauty without majesty . . . beautiful in a slight, dainty, or diminutive way,” (OED); but pretty as an adverb means “to a considerable extent” (OED), as in “it’s pretty far”!
These meanings are pretty far from one another!
The same pattern happens with fair. As an adjective, it means “beautiful” (OED), but its corresponding adverb fairly means “moderately” (OED) or “quite” (OED) as in “it’s fairly far.”
The Oxford English Dictionary doesn’t explain how these mysterious meanings arose, but it provides enough history to surmise a solution.
Fair is the older word, and that’s a clue! It meant “beautiful” in its Old English form in the year 888, its first recorded use. By contrast, pretty’s Old English form appears in 1000, but it means “cunning, crafty, wily, artful” (OED), and then vanishes from the written record until 1400 when it means “admirable,” especially as applied to a soldier’s warlike qualities. Its first use for the beauty of women appears in 1483.
But as pretty’s meaning was shifting from “artful” to “beautiful in a slight or dainty way” (perhaps by artful means), fair was broadening its sense of “beautiful” to include by the year 1240 “a large estate or fortune.”
Thus, by 1240, an ugly girl with a “fair fortune” was beautiful enough to be a fair match.
And pretty, perhaps inspired by fair’s broadening of its sense of beauty to include economic advantage, came to mean “considerable in number, quantity, or extent” (OED) in 1485, only two years after its first use for the beauty of women.
Thus, by 1485, an ugly girl with a “pretty fortune” was a pretty fair match.
So, that’s a pretty fair guess as to how pretty has come to mean both “beautiful” and “considerable.”
Pretty interesting, isn’t it?
* Orlando Bartro is the author of Toward Two Words, a comical & surreal novel about a man who finds yet another woman he never knew, usually available at Amazon for $4.91.