army (n.) Look up army at Dictionary.com
late 14c., "armed expedition," from Old French armée (14c.) "armed troop, armed expedition," from Medieval Latin armata "armed force," from Latin armata, fem. of armatus "armed, equipped, in arms," 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; the specific meaning "land force" first recorded 1786. Transferred meaning "host, multitude" is c.1500.

The Old English words were here (still preserved in derivatives like harrier), from PIE *kor- "people, crowd;" and fierd, with an original sense of "expedition," from faran "travel." 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.