sergeant (n.) Look up sergeant at Dictionary.com
c.1200, "servant," from Old French sergent, 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); cognate with Spanish sirviente, Italian servente.

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"), abolished 1880, from which Common Law judges were chosen; also used of certain other officers of the royal household. sergeant-major is from 1570s.