servient adj.

late 12c., serven, "to render habitual obedience to, owe allegiance to," also "minister, give aid, give help," from Old French servir "to do duty toward, show devotion to; set table, serve at table; offer, provide with," from Latin servire "be a servant, be in service, be enslaved;" figuratively "be devoted; be governed by; comply with; conform; flatter," originally "be a slave," related to servus "slave," which is of uncertain origin.

Perhaps from Etruscan (compare Etruscan proper names Servi, Serve, Latinized as Servius), but de Vaan says it is from Proto-Italic *serwo- "shepherd," *serwā- "observation," from PIE *seruo- "guardian" (source also of Avestan haraiti "heeds, protects"):

Rix 1994a argues that the original meaning of *serwo- probably was 'guard, shepherd', which underwent a pejorative development to 'slave' in Italy between 700 and 450 BC. Servire would be the direct derivative of servus, hence 'be a slave'; servare would in his view be derived from an older noun *serwa- or *serwom 'observation, heedance'.

It is attested by c. 1200 in widespread senses: "to be in the service of, perform a service for; attend or wait upon, be personal servant to; be a slave; owe allegiance to; officiate at Mass or other religious rites;" from early 13c. as "set food at table;" mid-14c. as "to wait on (customers)."

From late 14c. as "treat (someone or something) in some fashion." To serve (someone) right "treat as he deserves" is recorded from 1580s. Sense of "be useful, be beneficial, be suitable for a purpose or function" is from early 14c.; that of "take the place or meet the needs of, be equal to the task" is from late 14c.; that of "suffice" is from mid-15c.

The meaning "render active military service" is from 1510s. The sporting sense is attested by 1580s, first in tennis. The legal sense of "present" (a writ, warrant, etc.), "give legal notice of" is from early 15c. To serve hand and foot "minister to attentively" is by c. 1300.

He no schuld neuer wond

To seruen him fro fot to hond

["Amis and Amiloun," c. 1330]

c. 1200 (late 12c. as a surname), sergeaunt, also sergiaunte, serjainte, sergunt, cerjaunt, etc., "a servant, servingman," especially "an officer in a lord's retinue," from Old French sergent, serjant "(domestic) servant, valet; court official; soldier," from Medieval Latin servientum (nominative serviens) "servant, vassal, soldier" (in Late Latin "public official"), from Latin servire "to serve" (see serve (v.)).

The Latin word also is the source of Spanish sirviente, Italian servente. Sergeant is thus essentially a doublet of servant, and 16c. writers in English sometimes use the two words interchangeably.

By c. 1300 in a feudal sense of "tenant by military service under the rank of knight;" the modern military meaning "non-commissioned military officer" is recorded by 1540s. Originally a much more important position than after. As a police rank, in Great Britain from 1839.

The sense of "officer whose duty is to enforce judgments of a tribunal or legislative body" is from c. 1300 (hence sergeant at arms, attested from late 14c.). 

The Middle English alternative spelling serjeant (from Old French) was retained in Britain in special use as the title of a superior order of barristers from which Common Law judges were chosen (mid-14c.); in this use it is from the legal Latin phrase serviens ad legem, "one who serves (the king) in matters of law"). It was also used of certain other officers of the royal household.

Fem. form sergeantess is attested from mid-15c. Sergeant-major is attested from 1570s. The sergeant-fish (1871) so-called for lateral markings resembling a sergeant's stripes. Related: Sergeancy.