Sergeant
Middle English: from Old French sergent, from Latin servient- ‘serving’, from the verb servire . Early use was as a general term meaning ‘attendant, servant’ and ‘common soldier’; the term was later applied to specific official roles.
wiktionary
From Middle English sergeant, sergeaunt, serjent, serjaunt, serjawnt, sergant, from Old French sergeant, sergent, serjant, sergient, sergant(“sergeant, servant”), from Medieval Latin servientem, accusative of serviens(“a servant, vassal, soldier, apparitor”), from Latin serviēns(“serving”), present participle of serviō(“serve, be a slave to”). Doublet of servant and servient.
The fish is so called because of its stripes, supposed to resemble a sergeant's insignia of rank.
For the "er" being pronounced /ɑɹ/, see also clerk, derby, varsity.
etymonline
sergeant (n.)
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.
