Yes - actually it’s a contraction of does 'na ddim pwynt - so (doe)s ('n)a (ddi)m pwynt , quite a contraction! And indeed this is really the origin of the sa i… negative set - sa i’n gwybod is probably way back (nid oe)s (ohon)a i’n gwybod, then onward with so ti’n… (etc), which is (nid oe)s (ohon)ot ti’n...; similarly the SW set smo fi…, which includes the ddim instead of the nid: smo fi’n gwybod is probably (doe)s (di)m o(hono) fi’n gwybod.
Am sbort!
What fun!