What is the etymology for the French word "pas" as in "Je ne sais pas"?

Alaric's answer seems logical and feasible to me. I would just add that "pas" does have a cognate in at least 2 other Romance languages - in Occitan (Proven�al, Languedoc, etc) where it is used with verbs to mean not (without "ne": Parli pas - I do not speak; I vesi pas res - I don't see anything; Degun es pas vengut - nobody came), and in Catalan, where it is used as an emphatic negative: Jo no vull diner - I don't want money - but: Jo no vull pas diner I don't want *any* money at all).