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.).
      [1913 Webster]

            No fermenting must fills . . . the deep vats.
                                                  --Longfellow.
      [1913 Webster]

   2. [Cf. Musty.] Mustiness.
      [1913 Webster]

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.
      [1913 Webster]

   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.
      [1913 Webster]

            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.
         [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "must":
   absolute, allegiance, assigned task, bare necessities, be forced,
   be in for, be necessary, be obliged, binding, blast, blight,
   bounden duty, burden, burning, business, call for, call of duty,
   cancer, canker, cannot do otherwise, cannot help but, charge,
   commitment, committal, compulsory, conclusive, condition,
   de rigueur, decisive, deference, demand, demand for, demi-sec,
   desideration, desideratum, devoir, devotion, dictated,
   domestic wine, dry rot, duties and responsibilities, duty,
   entailed, essential, essentials, estral, estrous, estrual, ethics,
   extra sec, fealty, fetidity, fetidness, final, foulness, frowst,
   frowstiness, frowziness, fungus, funkiness, fustiness,
   hard-and-fast, have, have got to, have need to, have to, homage,
   hot, imperative, imperious, imported wine, imposed, in heat,
   in must, in rut, indispensable, irrevocable, light wine,
   line of duty, loyalty, malodorousness, mandated, mandatory, mildew,
   mission, mold, moldiness, moth, moth and rust, must item,
   mustiness, musty, necessaries, necessary, necessities, necessity,
   need, need for, need to, needs must, new wine, noisomeness,
   nonvintage wine, obligation, obligatory, occasion, odorousness,
   offensiveness, onus, ought, ought to, peremptory, pest, pink wine,
   place, prerequirement, prerequisite, prescript, prescriptive,
   putridness, rancidity, rancidness, rankness, reastiness, red wine,
   repulsiveness, required, requirement, requisite, requisition,
   respect, right, rose wine, rot, rottenness, rust, rutting, ruttish,
   rutty, sec, self-imposed duty, should, sine qua non, smelliness,
   smooth wine, smut, sparkling wine, still wine, stinkingness,
   stuffiness, sweet wine, the necessary, the needful, thin wine,
   ultimate, vin, vino, vintage wine, want, white wine, wine,
   without appeal, worm