late 14c., "tutelary or moral spirit" who guides and governs an individual through life, from Latin genius "guardian deity or spirit which watches over each person from birth; spirit, incarnation; wit, talent;" also "prophetic skill; the male spirit of a gens," originally "generative power" (or "inborn nature"), from PIE *gen(e)-yo-, from root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups.
The sense of "characteristic disposition" of a person is from 1580s. The meaning "person of natural intelligence or talent" and that of "exalted natural mental ability, skill in the synthesis of knowledge derived from perception" are attested by 1640s.
type of defensive opening in chess, 1935, in reference to Aron Nimzowitsch (1886-1935), Latvian-born Jewish chess genius who popularized a variation of the Indian defense (1884) attributed to Indian chess player Moheschunder Bannerjee.
"the 'genius' of a people, characteristic spirit of a time and place," 1851 (Palgrave) from Greek ēthos "habitual character and disposition; moral character; habit, custom; an accustomed place," in plural, "manners," from suffixed form of PIE root *s(w)e- third person pronoun and reflexive (see idiom). An important concept in Aristotle (as in "Rhetoric" II xii-xiv).
1580s, from 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. The oft-repeated quote "God is on the side of the largest battalions" (with many variants) usually is attributed to 17c. French military genius and marshal Turenne:
Madame, lui répondit-il, ne vous y fiez pas: j'ay tôujours vû Dieu do coté des gros Batallions. [E.Boursault, 1702]