Why is the ninth month called September?
early 14c., from Old French alarme (14c.), from Italian all'arme "to arms!" (literally "to the arms"). An interjection that came to be used as the word for the call or warning (cf. alert). Extended 16c. to "any sound to warn of danger or to arouse." Weakened sense of "apprehension, unease" is from 1833. Variant alarum is due to the rolling -r- in the vocalized form. Sometimes in early years anglicized as all-arm. Alarm clock is attested from 1690s (as A Larum clock).
a particular quivering sound of the silver trumpets to give warning to the Hebrews on their journey through the wilderness (Num. 10:5, 6), a call to arms, or a war-note (Jer. 4:19; 49:2; Zeph. 1:16).