Royal Navy slang dating 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!"