1580s, from Middle French bataillon (16c.), from Italian battaglione "battle squadron," from diminutive of Vulgar Latin battalia "battle," from Latin bauttere "to beat" (see batter (v.)). Specific sense of "part of a regiment" is from 1708.
Madame, lui répondit-il, ne vous y fiez pas: j'ay tôujours vû Dieu do coté des gros Batallions. [E.Boursault, 1702]