Merriam-Webster's Collegiate® Dictionary, 11th Edition
MUST
I. verb (present & past all persons must)
Etymology: 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 Date: before 12th century verb al auxiliary
1. be commanded or requested to , be urged to ; ought by all means to , be compelled by physical necessity to ,
3. be obliged to ; be compelled by social considerations to , be required by law, custom, or moral conscience to , be determined to , be unreasonably or perversely compelled to , be logically inferred or supposed to , be compelled by fate or by natural law to , was or were presumably certain to ; was or were bound to , may , shall , intransitive verb to be obliged to go ,
II. noun
Date: 1616
an imperative need or duty ; requirement , an indispensable item ; essential ,
III. noun
Etymology: Middle English, from Old English, from Latin mustum Date: before 12th century the expressed juice of fruit and especially grapes before and during fermentation,
IV. noun
Etymology: Middle English (Scots) moist, from Middle French must, alteration of musc musk
Date: 15th century
musk , mold , mustiness