suffix
Literary or formal suffix attached to the dictionary form of a verb to express prohibition. Equivalent to ～てはいけない or ～な, but with a classical, often written, tone. Commonly seen in proverbs, formal admonitions, and set phrases.
「油断する勿れ」と注意書きにあった。
The notice said, 'You must not be careless.'
「己の欲せざる所、人に施す勿れ」は論語の言葉だ。
'Do not do to others what you do not want done to yourself' is a saying from the Analects.
Irregular kanji usage; included for search coverage only.
な is the modern colloquial prohibitive suffix (e.g., 行くな 'don't go'), while 勿れ is its classical, literary counterpart used in formal writing and set phrases.
～てはいけない is the standard modern way to express prohibition in everyday speech, whereas 勿れ is a single suffix attached directly to the verb stem with a classical flavor.
Derived from the imperative form of the classical adjective 無し (なし, 'not exist'). This origin.