well-nigh vs nigh

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

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

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

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

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

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

well-nigh (adverb)	nigh (adverb)
(of actions or states) slightly short of or not quite accomplished; all but	near in time or place or relationship
	(of actions or states) slightly short of or not quite accomplished; all but