Yup, you're right and I was, let's say, 99% wrong. (The 1% comes from the fact that "you" and "vos" and "vous" are all related -- but "you" is a cousin of both,

Yup, you're right and I was, let's say, 99% wrong. (The 1% comes from the fact that "you" and "vos" and "vous" are all related -- but "you" is a cousin of both, not a descendant.) Still no way it's got anything to do with writing "thou" with a thorn at the front and misreading it as a "y", though.