nigh vs well-nigh

nigh is an adjective but well-nigh is not an adjective.

Word	Noun	Adjective	Verb	Adverb
nigh	No	Yes	No	Yes
well-nigh	No	No	No	Yes

As adverbs, nigh and well-nigh are synonyms defined as:

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

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

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

nigh (adverb)	well-nigh (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