Green’s Dictionary of Slang

(US) a combat infantryman; a Marine.

1978		J. Webb Fields of Fire (1980) 147: If I were a Colonel, I’d think it was a mutiny. A squad of snuffies trying to tell me how to run my perimeter.
1990		(con. 1965–73) G.R. Clark Words of the Vietnam War 246/2: The combat infantryman was known by many names such as [...] snuffies.
1993		L.A. Times 27 June 🌐 Back of them a couple of rows were the Snuffies, a cadre of Gus’ brothers-in-arms from the Vietnam days, all wearing their battle ribbons on sweaters or lapels, the five men who’d managed to attend representing a total of eight Purple Hearts.
2003		N.Y. Post 30 Mar. 12/5: Snuffy or Joe Snuffy – used by officers for the generic accident-prone privates.