fairly vs pretty

fairly is not an adjective while pretty is an adjective.

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

As adverbs, fairly and pretty are synonyms defined as:

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

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

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