fairly vs passably

fairly and passably both are adverbs.

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

As adverbs, fairly and passably are synonyms defined as:

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

fairly (adverb)	passably (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