side-arms n.

late Old English, "long, broad, spacious; extending lengthwise," from side (n.). Compare Old Norse siðr "long, hanging down." From late 14c. as "being from or toward the side," hence also "subordinate." Also "apart from the main course" of anything, as in side-road (1854); side-trip (1911). In side-eye (by 1922) the notion is "directed sideways."

[weapon], c. 1300, armes (plural) "weapons of a warrior," from Old French armes (plural), "arms, weapons; war, warfare" (11c.), from Latin arma "weapons" (including armor), literally "tools, implements (of war)," from PIE *ar(ə)mo-, suffixed form of root *ar- "to fit together." The notion seems to be "that which is fitted together." Compare arm (n.1).

The meaning "branch of military service" is from 1798, hence "branch of any organization" (by 1952). The meaning "heraldic insignia" (in coat of arms, etc.) is early 14c., from a use in Old French; originally they were borne on shields of fully armed knights or barons. To be up in arms figuratively is from 1704; to bear arms "do military service" is by 1640s.