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!"