It seemed that Kelsier was fluent in Spook’s street dialect, and even conversed with Spook in the dialect at one point. So I’m assuming Kelsier knew what Lestibournes really meant, and being who Kelsier was, giving him a new name probably was more about building the boy up, rather than just the length. That said, why Spook? If he was trying to boost Spook’s self confidence, why use a name like that?
It’s an inside joke between them. “Spook” means “Sneaky” or “Clever” in the street slang. It was a compliment.
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