Green’s Dictionary of Slang

(US) a pair of strong shoes suitable for rough use.

1942		Leatherneck (Nov.) 143: Boon Dockers – Not to be confused with Wedgies. They are field shoes weighing roughly 9,000,000 pounds (after a long march) [HDAS].
1944		R. Sherrod Tarawa 63: Betio would soon feel the tread of the U.S. Marines’ boondockers.
1953		L. Uris Battle Cry (1964) 37: The seabags became crowded within a barrage of skivvies, socks, overcoats, belts, boondockers, high-top dress shoes, field scarfs, and the rest of the wardrobe of a Marine.
1961		(con. early 1950s) J. Peacock Valhalla 229: He sat in his immaculate starched dungarees and spit-shined boondockers.
1972		J. Wambaugh Blue Knight 52: I’d land on that Dragon with both boondockers.
1997		Ol’ Army Joel’s Military Collectibles [Internet catalogue] US Armor Crewman. This is a uniform set and a nude figure. Uniform Set comes with a tankers helmet, goggles, a set of HBT overalls, boondockers, [...] a canteen, M-3 Grease Gun, M1911A1 and holster, a mag pouch, a first aid pouch, and a M-3 knife in M-6 scabbard.
2000		D. Richter Sun Stood Still xv: Amdt stayed close to the beach, about twenty yards out, because he still had his boondockers on. He couldn’t get them off.