nigh vs nearly

nigh is an adjective but nearly is not an adjective.

Word	Noun	Adjective	Verb	Adverb
nigh	No	Yes	No	Yes
nearly	No	No	No	Yes

As adverbs, nigh and nearly are synonyms defined as:

nigh and nearly: (of actions or states) slightly short of or not quite accomplished; all but

Other synonyms of nigh include about, almost, most, near, virtually, well-nigh.

nigh (adjective)	nearly (adjective)
not far distant in time or space or degree or circumstances
being on the left side

nigh (adverb)	nearly (adverb)
near in time or place or relationship	(of actions or states) slightly short of or not quite accomplished; all but
(of actions or states) slightly short of or not quite accomplished; all but	in a close manner