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."
1570s, "customer," short for obsolete chapman in its secondary sense "purchaser, trader" (also see cheap). Colloquial familiar sense of "lad, fellow, man or boy" is first attested 1716, usually with a qualifying adjective. Compare slang (tough) customer and German Kunde "customer, purchaser," colloquially "fellow."
c. 1400, reguler, "member of a religious order bound by vows," from regular (adj.) and from Medieval Latin regularis "member of a religious or monastic order." Sense of "soldier of a standing army" is from 1756. Meaning "regular customer" is by 1852; meaning "leaded gasoline" is by 1978; regular (adj.) in the sense of "unleaded" is by 1974.
also non-chalant, "indifferent, unconcerned, careless, cool," 1734, from French nonchalant "careless, indifferent," present participle of nonchaloir "be indifferent to, have no concern for" (13c.), from non- "not" (see non-) + chaloir "have concern for," ultimately from Latin calere "be hot" (from PIE root *kele- (1) "warm"). French chaland "customer, client" is of the same origin. Related: Nonchalantly.
late 14c., "one who lives under the patronage of another," from Anglo-French clyent (c. 1300), from Latin clientem (nominative cliens) "follower, retainer" (related to clinare "to incline, bend"), from PIE *klient-, a suffixed (active participle) form of root *klei- "to lean." The notion apparently is "one who leans on another for protection." In ancient Rome, a plebeian under the guardianship and protection of a patrician (who was called patronus in this relationship; see patron).
The meaning "a lawyer's customer" is attested from c. 1400, and by c. 1600 the word was extended to any customer who puts a particular interest in the care and management of another. Related: Cliency.
The relation of client and patron between a plebeian and a patrician, although at first strictly voluntary, was hereditary, the former bearing the family name of the latter, and performing various services for him and his family both in peace and war, in return for advice and support in respect to private rights and interests. Foreigners in Rome, and even allied or subject states and cities, were often clients of Roman patricians selected by them as patrons. The number of a patrician's clients, as of a baron's vassals in the middle ages, was a gage his greatness. [Century Dictionary]