adverb
Archaic negative adverb used in classical or highly literary contexts; often paired with prohibitive or negative expressions. Not used in modern everyday speech.
ゆめ疑うことなかれ。
Never doubt it.
ゆめ忘るることなかれ。
By no means forget it.
The exact derivation is uncertain; it is an old negative adverb found in classical Japanese texts.