From Bouvier's Law Dictionary, Revised 6th Ed (1856) :
LEY. This word is old French, a corruption of loi, and signifies law; for example, Termes de la Ley, Terms of the Law. In another, and an old technical sense, ley signifies an oath, or the oath with compurgators; as, il tend sa ley aiu pleyntiffe. Brit. c. 27.