Yours, Etc.: Origins and Uses of 8 Sign-Offs
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]