Synonyms for Well-nigh:

adv

almost (adverb)
all but.

adv.

almost (adverb)
all but, nigh, just about, virtually, most, about.
just about (adverb)
just about, nearly.
most (adverb)
almost.
nearly (adverb)
nearly.

n.

practically (noun)
almost completely.
well-nigh (noun)
almost, just about, nigh, most, about, nearly, virtually, near, all but.

Other synonyms:

adv
almost, most.