Why is the ninth month called September?
Old English moste, past tense of motan "have to, be able to," from Proto-Germanic *mot- "ability, leisure (to do something)" (cf. 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- "to measure, to take appropriate measures" (see medical (adj.)). Used as present tense from c.1300, 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).
"mold," c.1600, perhaps a back-formation of musty (q.v.).
"male elephant frenzy," 1871, 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 is from 1959.