Etymology

nigh (adv.)

"near, nearby, close together, adjacent," Middle English neigh, from Old English neah (West Saxon, Kentish), neh (Anglian), from Proto-Germanic *naehwa- (source also of Old Saxon nah, Old Frisian nei, Middle Dutch, Dutch na, Old High German nah, German nah, Gothic nehwa), of uncertain origin, with no cognates outside Germanic. The Old English progression was neah - near - niehsta, for "nigh - nigher - nighest." But the comparative near and the superlative nehst (see next) gradually evolved into separate words that were no longer felt as related to nigh. New comparative and superlative forms nigher, nighest developed 14c. as phonetic changes obscured the original relationships. As an adjective and preposition in Middle English.

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Definitions of nigh
1
nigh (adv.)
near in time or place or relationship;
The end draws nigh
Synonyms: near / close
nigh (adv.)
(of actions or states) slightly short of or not quite accomplished; all but;
the recording is well-nigh perfect
talked for nigh onto 2 hours
Synonyms: about / almost / most / nearly / near / virtually / well-nigh
2
nigh (adj.)
being on the left side;
the near or nigh horse is the one on the left
the animal's left side is its near or nigh side
Synonyms: near
nigh (adj.)
not far distant in time or space or degree or circumstances;
Synonyms: near / close
From wordnet.princeton.edu