auxiliary of prediction, "be obliged, be necessarily impelled," from Old English moste, past tense of motan "have to, be able to," from Proto-Germanic *motanan (source also of Old Saxon motan "to be obliged to, have to," Old Frisian mota, Middle Low German moten, Dutch moeten, German müssen "to be obliged to," Gothic gamotan "to have room to, to be able to"), perhaps from PIE root *med- "take appropriate measures," but this old suggestion lately has been doubted. Used as present tense from c. 1300, eventually displacing motan, from the custom of using past subjunctive as a moderate or polite form of the present.
"new wine," Old English must, from Latin mustum (also source of Old High German, German most, Old French moust, Modern French moût, Spanish, Italian mosto), short for vinum mustum "fresh wine," neuter of mustus "fresh, new, newborn," perhaps literally "wet," and from PIE *mus-to-, from root *meus- "damp" (see moss).
"male elephant frenzy," 1878, from earlier adjective (1855), from Urdu mast "intoxicated, in rut," from Persian mast, literally "intoxicated," related to Sanskrit matta- "drunk, intoxicated," past participle of madati "boils, bubbles, gets drunk," from PIE root *mad- "wet, moist" (see mast (n.2)).
"that which has to be done, seen, or experienced," 1892, from must (v.). As an adjective, "obligatory, indispensable," by 1912, from the noun; must-read (n.) is from 1959.