The word is derived from a Greek word “toxikón”, which is a the arrow of type of bow.
It is thought that this word became our western world meaning of “poisonous” because of the story of Hercules.
Hercules’ second labor was to kill the nine-headed Hydra.
Once Hercules slayed the Hydra, he dipped his arrows in the Hydra’s blood. This made his arrows poisonous.