Green’s Dictionary of Slang

impossible.

1908		‘Doss Chiderdoss’ ‘When Woman’s Tongue Wags’ Sporting Times 25 July 1/4: You are right when you hold that it’s almost ‘impos.’ / A good lady’s jaw action to stay.
1911		Gem 11 Nov. 2: Yaas, but it’s imposs!
1924		Galsworthy White Monkey 71: My dear! Impos!