The Collaborative International Dictionary of English v.0.48:

Must \Must\, n. [AS. must, fr. L. mustum (sc. vinum), from
   mustus young, new, fresh. Cf. Mustard.]
   1. The expressed juice of the grape, or other fruit, before
      fermentation. "These men ben full of must." --Wyclif (Acts
      ii. 13.).
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            No fermenting must fills . . . the deep vats.
                                                  --Longfellow.
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   2. [Cf. Musty.] Mustiness.
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The Collaborative International Dictionary of English v.0.48:

Must \Must\ (m[u^]st), v. i. or auxiliary. [OE. moste, a pret.
   generally meaning, could, was free to, pres. mot, moot, AS.
   m[=o]ste, pret. m[=o]t, pres.; akin to D. moetan to be
   obliged, OS. m[=o]tan to be free, to be obliged, OHG. muozan,
   G. m["u]ssen to be obliged, Sw. m[*a]ste must, Goth.
   gam[=o]tan to have place, have room, to able; of unknown
   origin.]
   1. To be obliged; to be necessitated; -- expressing either
      physical or moral necessity; as, a man must eat for
      nourishment; we must submit to the laws.
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   2. To be morally required; to be necessary or essential to a
      certain quality, character, end, or result; as, he must
      reconsider the matter; he must have been insane.
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            Likewise must the deacons be grave.   --1 Tim. iii.
                                                  8.
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            Morover, he [a bishop] must have a good report of
            them which are without.               --1 Tim. iii.
                                                  7.
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   Note: The principal verb, if easily supplied by the mind, was
         formerly often omitted when must was used; as, I must
         away. "I must to Coventry." --Shak.
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The Collaborative International Dictionary of English v.0.48:

Must \Must\, v. t. & i.
   To make musty; to become musty.
   [1913 Webster] must

The Collaborative International Dictionary of English v.0.48:

must \must\, musth \musth\(m[u^]st), a. [Hind. mast intoxicated,
   ruttish, fr. Skr. matta, p.p. of mad to rejoice, intoxicate.]
   (Zool.)
   Being in a condition of dangerous frenzy, usually connected
   with sexual excitement; -- said of adult male elephants which
   become so at irregular intervals, typicaly due to increased
   testosterone levels. -- n.
   (a) The condition of frenzy.
   (b) An elephant in must.
       [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

must
    adj 1: highly recommended; "a book that is must reading"
    n 1: a necessary or essential thing; "seat belts are an absolute
         must"
    2: grape juice before or during fermentation
    3: the quality of smelling or tasting old or stale or mouldy
       [syn: mustiness, must, moldiness]