This is another entry about language.
I just found out that the usage of “pretty” to mean “fairly” has been around since the 1500s. So why is it still considered informal? I don’t know. Weird that it’s maintained the same diction level for 400 years.
The qualifying adverb pretty, meaning “fairly or moderately” has been in general use since the late 16th century. Although most common in informal speech and writing, it is far from restricted to them, and often is less stilted than alternatives such as relatively, moderately, and quite.
I questioned whether maybe “pretty” as “fairly” was closer to original meaning, but when I looked further, I saw that this usage has apparently always been slang-ish or not denotative:
bef. 1000; ME prati(e), pratte, prettie, cunning, gallant, fine, handsome, pretty; OE prættig, prettī, cunning, deriv. of prǣtt, a trick, wile (c. D part, pret, trick, prank, ON prettr, trick, prettugr, tricky)
I could possibly get some more insight by looking into the attractiveness connotation of the word “fair.” “Monday’s child is fair of face.” I have wondered about that word a lot recently anyway. Will follow up.