Green’s Dictionary of Slang

stiff v.1

[? the response, ‘I say, that’s a bit stiff,’ to such an outburst]

to curse, to swear.

[UK]Kipling ‘The Arrest of Lieutenant Golightly’ in Plain Tales from the Hills (1890) 131: I’ve ’eard a few beggars in the clink blind, stiff and crack on a bit.
[UK]Wodehouse Mating Season 98: Nor could one fairly blame them for blinding and stiffing.
[UK]Wodehouse Jeeves in the Offing 33: Those solid citizens have to learn to curb the tongue. Creates a bad impression, I mean, if they start blinding and stiffing.