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!"
On roads, lengthsmen were responsible for a few miles between adjacent villages and their duties were to lay lengths of pipe for local female villagers.