Royal Navy slangdating back before the Second World War, it means to defecate, crap, shit or take a dump. Nothing fancy, to crimp off a length you merely have to evacuate your bowels.
"That fat fucker of a stoker is getting shitty with me again!"
"What're you goin' to do?"
"Next time the cunt is down the engine room, I'll crimp off a length in his best boots!"
And dying in yo' bed many years from now, would ya be willing ta trade all da days from dis here day ta dat fo' one chance, just one chance, ta come back here as young men an' tell our enemies dat dey may take our lives but dey will never take our freedom ... YUB NUB!