Green’s Dictionary of Slang

extra, superfluous, spare.

1945		D. Bolster Roll On My Twelve 134: I see a lump o’ nutty goin’ gash, so I wolfs it.
1955		B. Hill Boss of Britain’s Underworld 123: He was letting the boys take away lorry-loads of gash parachutes.
1962		H. Livings Nil Carborundum (1963) Act III: We get no rations for gash cups of tea, Taffy. Gerrout.
1971		(con. WWII) B. Aldiss Soldier Erect 212: We won’t get cracking until all the gash bods [...] are sent back to base.
1984		(con. WW2) T. Jones Heart of Oak [ebook] Meal scraps were scraped into the gash bucket by each man after he finished his meal [...] We ditched the gash over the side straight down a foul-smelling garbage chute.
2007		S. Low Boys from Baghdad 108: [T]he houseboys drove off with the ‘gash’ items.
2009		R. Milward Ten Storey Love Song 178: Their band Whirlwind are absolutely gash.