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!"