1580s, "system of military discipline," from Latin militia "military service, warfare," from miles "soldier" (see military). Sense of "citizen army" (as distinct from professional soldiers) is first recorded 1690s, perhaps from a sense in French cognate milice. In U.S. history, "the whole body of men declared by law amenable to military service, without enlistment, whether armed and drilled or not" (1777).