From what I can gather, this is what’s called a “conversion word.” Where the word itself (its “form”) doesn’t change, but the word’s part of speech does. Before the mid 1500s, pretty (also spelled pretie, prety, and prettie) was only an adjective. By end of the century, it started to be used as an adverb (the usage you’re describing) as well. Per the OED, one use of pretty as an adjective meant “considerable, sizable. A pretty many” (meaning a good many, or a lot). It was used this way from the 1400s on. In the mid to late 1500s it shifted. There’s a use from 1577: “Berlady tis prety good meate.” And then in 1598 a usage that’s definitely connected to the one you’re talking about: “Boccace is prettie hard, yet understood: Petrarche harder but explaned.” It was likely “converted” verbally and then the written word caught up. I know that’s not the most complete answer, but hopefully it’s satisfying enough? I’m just a word nerd who recently discovered most public library cards provide online access to the Oxford English Dictionary, so if any experts see this and know I’ve gotten something wrong, feel free to correct me! :) Edited for grammar and clarity Answer from toukacottontails on reddit.com