c. 1200, servaunt, "male or female personal or domestic attendant, one owing duty of service to a master or lord, one employed by another and subject to his orders," from Old French servant "servant; foot-soldier," noun use of servant "serving, waiting," present participle of servir "to attend, wait upon" (see serve (v.)).

From early 14c. as "a slave," also used of bees. In North American colonies and in U.S., it was the usual designation for "slave" 17c.-18c. (in 14c.-15c. and later in Biblical translations the word often was used to render Latin servus, Greek doulos "slave").

Also in Middle English "professed lover, one devoted to the service of a lady" (mid-14c.).  In 14c.-16c. sometimes confused with sergeant.  Public servant is attested from 1670s. Wyclif (late 14c.) has servauntesse "female slave, maidservant, handmaiden."

also maid-servant, "female servant," 1520s, from maid (n.) + servant.

also man-servant, "a man who is a servant," late 14c., from man (n.) + servant.

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.