Why is the ninth month called September?
"arms and weapons collectively," c.1300; see arm (n.2) + -ory. Meaning "place where arms are manufactured" is from mid-15c. Also used in a sense of "arsenal" (mid-15c.), "the science of heraldry" (late 15c.), from Old French armoierie, from armoier "to blazon," from Latin arma "weapons" (see arm (n.2)).