From Middle English sergeant, sergeaunt, serjent, serjaunt, serjawnt, sergant, from Old French sergeant, sergent, serjant, sergient, sergant, from Medieval Latin servientem, accusative of serviens, from Latin serviēns ("serving"), present participle of serviō. servant, and servient.
The fish is so called because of its stripes, supposed to resemble a sergeant's insignia of rank.