I assume it's an ethnic slur but the origin is still interesting... for instance, only certain areas/people use "jew" to mean "bargain" - I heard a friend say it about Judas in The Passion, I think it was. He called it jewing and I was like, you mean, bargaining?

But welshing is specific - that is, can one welsh on anything *other* than a bet? There are other things not paid/done when they should be, one doesn't "welsh" on, say, a debt racked up at the gym, does one? Even if it's never paid?