Entry in progress—B.P.
 
Wiktionary: grunt work
Etymology
(Can this etymology be sourced?) From military jargon.
Noun
grunt work (uncountable)
1.(idiomatic) Work (especially that which is heavy, repetitive or mindless) that is considered undesirable and therefore delegated to underlings.
 
Urban Dictionary
grunt
Grunt is an acronym used during WW2 for troops who had no formal training, or skills. G-general, R-replacement, UNT-untrained,...GRUNT, as they had no special training, they were given rifles and sent to the front.
Grunts do all the fighting.
by Paul Manning Aug 6, 2003
 
Snopes.com Message Board
snopes
29 March 2009, 03:59 PM
Grunt = Ground Recruit Usually Not Trained
Comment: A short question for you. I have been told, since I have been a soldier that the term “GRUNT” is an old army acronym for Ground Recruit Usually Not Trained. Is there any way that you could find out. I was in the infantry in the mid 90’s and was told it originated in WWII so that commanders getting new recruits to the front lines would know if they had been to the school of the infanry(now at Ft. Benning, GA.).
   
Google Books
Walking with Grunts:
An Australian Army Chaplain with the 8th Infantry ...
By Father Stan Hessey
Xlibris Corporation (Xlibris.com.au)
2011
Pg. 30:
“Grunt” is an American term, which appeared in US Training Units and stood for General Rifleman Usual Not Trained. G.R.U.N.T. It came to Vietnam late, being first used in print in 1969.