army:  Latin armāta ‘armed’, the past participle of the verb armāre, was used in postclassical times as a noun, meaning ‘armed force’. Descendants of armāta in the Romance languages include Spanish armada and French armée, from which English borrowed army. In early usage it could (like Spanish armada) mean a naval force as well as a land force (‘The King commanded that £21,000 should be paid to his army (for so that fleet is called everywhere in English Saxon) which rode at Greenwich’, Marchamont Needham’s translation of Selden’s Mare clausum 1652), but this had virtually died out by the end of the 18th century. => arm, armada
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.