Green’s Dictionary of Slang

(US, orig. milit.) a US combat infantryman in Vietnam.

1967		D. Reed Up Front 3: In Vietnam he [i.e. the infantryman] goes by an assortment of names — the Grunt, Boonie Rat, Line Dog, Ground Pounder, Hill Humper, or Jarhead [HDAS].
1969		R.W. Jordan ‘The Boonie Rat Ballad’ 🎵 Boonie Rat, Boonie Rat, / Scared but not alone, / Today I see my Freedom Bird, / Today, I’m going home.
1981		(con. Vietnam War) A. Santoli Everything We Had (1982) 167: That [mood] permeated all the way down to Snuffy in the field. Not just my guys, but Boonie Rat out there with a commissioned unit.
1990		G.R. Clark Words of the Vietnam War 246/2: The combat infantryman was known [...] as bluelegs, snuffies, grunt, boonie rat, ground pounder, crunchies, legs, beetle stompers, bush beetles, line doggies and doggies.
1999		A. Dodson posting at Sights and Sounds from the War in Vietnam 🌐 I served in II corps Vietnam 1969 to 1971, with 2/1 Cavalry (Blackhawks) a mechanized infantry unit. [...] listening to your tapes sure brought back beaucoup memories. Especially the radio traffic between us folk in the boonies, the artillery guys, and/or the helicopter gunships. [...] Thanks again for making an Old Boonie Rat’s day. God bless you.