Archaic negative adverb used in classical or highly literary contexts; often paired with prohibitive or negative expressions. Not used in modern everyday speech.
Never doubt it.
By no means forget it.
Archaic adverb usually written in kana; the kanji form 努 is rare and may be confused with other readings.
Kanji form is archaic and rarely used; learners should recognize it but use the kana spelling.
The exact derivation is uncertain; it is an old negative adverb found in classical Japanese texts.