Again the problem of definition. Just what is cbw.
Chemical weapons are a different problem.
Poisons were certainly used in individual cases and there were recorded incidents in both the ancient and modern world of wells being intentionally poisoned. There is a problem with this being considered terrorism however since this seems to have been done for its military utility rather than to terrorize the victims or third parties. Biological warefare was certainly present at least by the latter middle ages if one includes throwing dead and diseased bodies into beseiged areas. It must be stressed that there was no real knowledge of the cause of disease at this time. Prevailing opinion seems to have been that the bad air was somehow responsible.