customer (n.)

late 14c., custumer, "customs official, toll-gatherer;" c. 1400, "one who purchases goods or supplies, one who customarily buys from the same tradesman or guild," from Anglo-French custumer, Old French coustumier, from Medieval Latin custumarius "a toll-gatherer, tax-collector," literally "pertaining to a custom or customs," a contraction of Latin consuetudinarius, from consuetudo "habit, usage, practice, tradition" (see custom (n.)).

The more generalized meaning "a person with whom one has dealings" emerged 1540s; that of "a person to deal with" (usually with a defining adjective: tough, etc.) is by 1580s. In Shakespeare, the word also can mean "prostitute."