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!"
Leng is a word that describes something SUPER attractive. It's a leveled up version of peng and both words are roadman slang, used on the streets of Britain. It can be used for males, female, or even inanimate objects, or just to describe something that's reaaaaalllly good. If you want to use leng, then the thing/person has to REALLY REALLY be attractive.
"OMG , Samir is SO leng..."
"Raaah fam this pizza is WAY lenger than your crusty dusty sandwich over there!"
"Bro that girl is lenggggg."
"This drawing is so leng."