pretty vs fairly

pretty is an adjective but fairly is not an adjective.

Word	Noun	Adjective	Verb	Adverb
pretty	No	Yes	No	Yes
fairly	No	No	No	Yes

As adverbs, pretty and fairly are synonyms defined as:

Other synonyms of pretty include jolly, middling, moderately, passably, reasonably, somewhat.

pretty (adjective)	fairly (adjective)
(used ironically) unexpectedly bad
pleasing by delicacy or grace; not imposing

pretty (adverb)	fairly (adverb)
to certain extent or degree	without favoring one party, in a fair evenhanded manner
	to certain extent or degree
	in conformity with the rules or laws and without fraud or cheating