late 14c., "armed expedition," from Old French armée "armed troop, armed expedition" (14c.), from Medieval Latin armata "armed force," from Latin armata, fem. of armatus "armed, equipped, in arms," as a noun, "armed men, soldiers," past participle of armare "to arm," literally "act of arming," related to arma "tools, arms" (see arm (n.2)).
Originally used of expeditions on sea or land; restriction to "land force" is by late 18c. Transferred meaning "host, multitude" is c. 1500. Meaning "body of men trained and equipped for war" is from 1550s.
The Old English words were here (still preserved in derivatives such as harrier; see harry (v.)), from Proto-Germanic *harjan, from PIE *korio- "people, crowd;" and fierd, with an original sense of "expedition," from Proto-Germanic *farthi-, related to faran "travel" (see fare (v.)). In spite of etymology, in the Anglo-Saxon Chronicle here generally meant "invading Vikings" and fierd was used for the local militias raised to fight them. Army-ant is from 1863.