entry
mustn't
/ˈmʌsənt/is not permitted or is necessarily forbidden
From Proto-Germanic, through Old and Middle English must (be obliged) + O.English not (in no way).
Word Ancestry
English speakers inherited must from a word that originally covered a surprisingly broad territory: being allowed, being able, and being obliged. Old English mōtan could mean “have the opportunity to,” while its past form mōste eventually became the modern present must—a polite grammatical shortcut that took over around 1300. The negative half, not, began as Old English nāht or noht, literally something like “in no way,” then wore itself down through rapid speech. By the mid-18th century, must not had compressed into mustn't, a tiny verbal traffic sign: stop, forbidden, no exceptions. Say it aloud and the vanished “o” is still faintly there—the English language has turned a full prohibition into two clipped beats.
The Story
English speakers inherited must from a word that originally covered a surprisingly broad territory: being allowed, being able, and being obliged. Old English mōtan could mean “have the opportunity to,” while its past form mōste eventually became the modern present must—a polite grammatical shortcut that took over around 1300. The negative half, not, began as Old English nāht or noht, literally something like “in no way,” then wore itself down through rapid speech. By the mid-18th century, must not had compressed into mustn't, a tiny verbal traffic sign: stop, forbidden, no exceptions. Say it aloud and the vanished “o” is still faintly there—the English language has turned a full prohibition into two clipped beats.
Kin & Kindred
From 'must'·be obliged, be necessarily impelled, or have to
Cognates
Related words in other languages
Derived Terms
English words from this root
From 'not'·in no way; negative particle
Derived Terms
English words from this root