Old English grunnettan "to grunt," frequentative of grunian "to grunt," probably imitative (compare Danish grynte, Old High German grunnizon, German grunzen "to grunt," French grogner, Latin grunnire "to grunt"). Related: Grunted; grunting. Grunter "a pig" is from 1640s.
1550s, from grunt (v.); as a type of fish, from 1713, so called from the noise they make when hauled from the water; meaning "infantry soldier" emerged in U.S. military slang during Vietnam War (first recorded in print 1969); used since 1900 of various low-level workers. Grunt work first recorded 1977.