Green’s Dictionary of Slang

(US milit.) the lowest rank of private soldier, thus buck sergeant, the lowest level of sergeant, on the lowest pay grade .

1899–1900		C.L. Cullen Tales of the Ex-Tanks 103: That’s what I was – a buck private.
1905		‘West Point Sl.’ in Howitzer (US Milit. Academy) 292–5: Buck – A private in the ranks.
1918		C. Richmond Somewhere in France 🌐 A corporal let his rifle get knocked into a well and was busted to a buck.
1919		H.C. Witwer Smile A Minute 95: They claimed what right had he to order them around when they was all brothers now and a general was the same as a buck private.
1927		W. Edge Main Stem 8: May even go in [the Army] as a buck private.
1939		Army and Navy Register (US) 18 Nov. 3/2: ‘Buck private,’ a private without any specialist’s rating.
1948		(con. 1944) N. Mailer Naked and Dead 65: Martinez is a buck private in 1937.
1953		Randolph & Wilson Down in the Holler 104: I have seen a mature woman, mother of a large family, burst out laughing when a former soldier spoke casually of buck privates.
1961		G.L. Coon Meanwhile, Back at the Front (1962) 200: I’m going to bust you so far you’ll have to climb a tree to see a buck private’s ass!
1977		(con. 1965-66) P. Caputo Rumor of War 50: A big buck sergeant strode past our truck and called out, ‘Hey, second goddamn platoon, you all ready for them Veet-Cong?’.