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]