c.1200, "servant," 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 servientem "serving," present participle of servire "to serve" (see serve (v.)); cognate with Spanish sirviente, Italian servente; a twin of servant, and 16c. writers sometimes use the two words interchangeably.
Specific sense of "military servant" is attested from late 13c.; that of "officer whose duty is to enforce judgments of a tribunal or legislative body" is from c.1300 (sergeant at arms is attested from late 14c.). Meaning "non-commissioned military officer" first recorded 1540s. Originally a much more important rank than presently. As a police rank, in Great Britain from 1839.
Middle English alternative spelling serjeant (from Old French) was retained in Britain in special use as title of a superior order of barristers (c.1300, from legal Latin serviens ad legem, "one who serves (the king) in matters of law"), from which Common Law judges were chosen; also used of certain other officers of the royal household. sergeant-major is from 1570s. The sergeant-fish (1871) so-called for lateral markings resembling a sergeant's stripes. Related: Sergeancy.