a person who purchases goods or services from another; buyer; patron.
2.
Informal. a person one has to deal with: a tough customer; a cool customer.
Origin: 1400–50; late Middle English; see custom, -er1; compare Middle English customer collector of customs < Anglo-French; Old French costumier, cognate with Medieval Latin custumārius;see customary
14c., "customs official;" later "buyer" (early 15c.), from Anglo-Fr. custumer, from M.L. custumarius, from L. consuetudinarius (see custom). More generalized meaning "a person with whom one has dealings" emerged 1540s; that of "a person to deal with" (usually wth an adjective,