Английский Этимологический словарь - lay (adj.)

- c.1300, from O.Fr. lai "secular, not of the clergy," from L.L. laicus, from Gk. laikos "of the people," from laos "people." In M.E., contrasted with learned, a sense revived in 19c. usage for "non-expert." Layman "outsider, non-expert" (especially in regards to law or medicine) is from 1477.