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As for William: sometimes he was called Mamzer because at that times those who wrote that used this word for bastards. These guys were Jewish. That doesn't mean he was Jewish. It's liek if I called him fattyú that wouldn't make him Hungarian.
> In Europe, it may have entered the vernacular
Did it? I would expect good Catholics distance themselves of Jewish expressions, especially in those times. And (many) hundreds of years later it doesn't matter.