Green’s Dictionary of Slang

1. (US) a large shoe, usu. in pl.; occas. attrib.

1863		US Army and Navy Journal I 180/2: Expensive shoes [...] are often thrown away unused, for the despised Government ‘mudscows.’ These ‘mudscows’ or ‘gunboats’ [...] are low-cut, stiched, very light, and very cheap [...]. The sole is very broad, and the heels broad and low [DA].
1886		Lantern (New Orleans, LA) 27 Oct. 4: Gunboats are no longer fashionable for ladies’ walking shoes in Chicago.
1913		Polly & Her Pals 25 Nov. [synd. cartoon] Take them gunboats off that sofy pillar, y’young hyena!
1920		C.B. Booth ‘Mr Clacksworthy Tells the Truth’ Detective Story 19 Oct. 🌐 All th’ strap-hangers must wear iron cleats on their gunboats.
1928		R. Carr Rampant Age 126: Say, Mom, how about my havin’ a pair of new shoes? These old gunboats are all scuffed up.
1932		(con. 1910s) J.T. Farrell Young Lonigan in Studs Lonigan (1936) 30: Reardon nodded as he shifted his weight from the right to the left gunboat.
1941		B. Schulberg What Makes Sammy Run? (1992) 61: Get those gunboats out of dry dock and get the hell over here.
1952		(con. 1861-5) B.I. Wiley Life of Billy Yank 59: Rough black shoes, known in soldier parlance as gunboats.
1977		(con. 1908) J. Monaghan Schoolboy, Cowboy, Mexican Spy 35: Shabby ‘catalog shoes’ – a cheap pattern with the ‘gunboat toes’ worn in Philadelphia.

2. (US) an armed stage-coach.

1878

A.F. Mulford Fighting Indians 66: The new Black Hills stages or gunboats [...] consist of a very heavy and large stage with a 2-pound Mountain Howitzer on top.

3. (US tramp) a water bucket made from a gallon can [its unwieldiness].

1929		V.W. Saul ‘Vocab. of Bums’ in AS IV:5 340: Gun-boat—An empty gallon-size tin can.
1931		Irwin Amer. Tramp and Und. Sl. 93: Gunboat.– [...] An empty tin can, used in the ‘jungles’ for cooking, carrying water or liquor, or for boiling the clothes.
1949		Monteleone Criminal Sl. (rev. edn) 109: gunboat A tin can used by tramps to cook in.
1976		M. Braly False Starts 115: We were supplied with a gunboat of water.

4. (US tramp) a steel coal wagon.

1931		Irwin Amer. Tramp and Und. Sl. 93: Gunboat.—A steel coal car.
1942		Berrey & Van den Bark Amer. Thes. Sl.