means An auxiliary verb expressing necessity, obligation, or near-certain inference — what has to be, what is required, or what almost surely is the case.

from From Old English 'moste,' the past tense of 'motan' (to be able to, to be allowed to), which over time hardened from 'may' into 'must.' It's a cousin of German 'müssen.' Oddly, the word began life talking about permission and possibility, then drifted into pure obligation — the freedom of 'may' curdling into the compulsion of 'must.' (The 'must' meaning new wine, and the 'must' of mustiness, are entirely separate words.)