Why is the ninth month called September?
c.1200, a shortening of Old French escarn "mockery, derision, contempt," a common Romanic word (cf. Spanish escarnio, Italian scherno) of Germanic origin, from Proto-Germanic *skarnjan "mock, deride" (cf. Old High German skern "mockery, jest, sport," Middle High German scherzen "to jump with joy").
Probably influenced by Old French escorne "affront, disgrace," which is a back-formation from escorner, literally "to break off (someone's) horns," from Vulgar Latin *excornare (source of Italian scornare "treat with contempt"), from Latin ex- "without" (see ex-) + cornu "horn" (see horn (n.)).
c.1200, from Anglo-French, Old North French escarnir (Old French escharnir), from the source of scorn (n.). Cf. Old High German skernon, Middle Dutch schernen. Related: Scorned; scorning. Forms in Romanic languages influenced by confusion with Old French escorner "deprive of horns," hence "deprive of honor or ornament, disgrace."