ORIGIN OF MUST - Middle English moste, from Old English mōste, past indicative & subjunctive of mōtan to be allowed to, have to; akin to Old High German muozan to be allowed to, have to First Known Use: before 12th century

Definition of must (verb) \məs(t), ˈməst\

  • used to say that something is required by a rule or law ("You must present your passport.")

  • used to say that someone should do something ("We must hurry to the airport.")

  • used to say that something is very likely ("You must be John.")

Which meaning developed first in the English Language, "strong obligation" or "certainty"?

  • Tricky question...My impression is that the meaning expressing 'highly likely' developed along with the use of must meaning 'obligation'... Let's see if users have more on this issue!!
    – user66974
    Commented Nov 3, 2014 at 13:23
  • Considering the etymology, it seems like the obligation sense must have come first [pun intended]. The probability sense seems more metaphorical, and metaphors are usually secondary meanings.
    – Barmar
    Commented Nov 3, 2014 at 20:58
  • @Barmar You ask a serious question and all you get is 45 views and no answer. That's very motivating.
    – Centaurus
    Commented Nov 6, 2014 at 16:14
  • Sorry, I'm not a language historian. The best I can do is a guess, and I don't think it's definitive enough for an answer.
    – Barmar
    Commented Nov 6, 2014 at 16:17

1 Answer 1


Historically, "strong obligation" (the deontic reading) arose before "certainty" (the epistemic reading). The deontic readings are attested in the earliest Old English texts, but the epistemic reading didn't arise until considerably after the Old English period.

In fact, the pathway of change from deontic to epistemic modality is a very common tendency cross-linguistically, and the reverse is said to be rare. Traugott (1989) has examples and discussion from the history of English.

  • After being ignored for a month, someone decided to answer. Great.
    – Centaurus
    Commented Nov 30, 2014 at 0:42
  • 1
    Sorry I wasn't quicker; I'm only in here every now and then. Commented Nov 30, 2014 at 0:43

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