The Sn for tin is from "stannum," as the Pb for lead comes from "plumbum." I think I know the derivation of plumbum - from using it to determine the true vertical (plumb), as a weight on a "plumb bob" because it's heavy/dense - but haven't a clue to how stannum might have arisen. At least there's a starting point.

Not that that helps us to know where "tin" came from. (Do we know where "lead" came from, for that matter?)