late 13c., "to join in marriage," from Old French alier "combine, unite," from a differentiated stem of aliier (from Latin alligare "bind to;" see alloy). Meaning "to form an alliance, join, associate" is late 14c. Related: allied; allying.
late 14c., "relative, kinsman," from ally (v.); mid-15c. in the sense of "one united with another by treaty or league."