c.1300, pp. adj. from ally (q.v.). Originally of kindred; in ref. to league or formal treaty, it is first recorded late 14c.
c.1300, from O.Fr. alier "combine, unite," from a differentiated stem of aleier (source of alloy), from L. alligare "bind to" (see alloy). The noun is c.1600 in the sense of "one united with another by treaty or league," from the verb.