Let's break it down:
* Poisonous:
* Poison: A substance that can cause illness or death when ingested, inhaled, or absorbed.
* -ous: Full of, having the quality of.
* Poisonous: Having the quality of poison, capable of causing harm or death.
* Treacherous:
* Treachery: Betrayal of trust, deceitfulness, or disloyalty.
* -ous: Full of, having the quality of.
* Treacherous: Full of treachery, deceitful, unreliable, and dangerous.
In both cases, "-ous" transforms the base words into adjectives that describe something possessing the characteristic of the base word. It signifies that the thing in question is capable of causing harm or danger in some way.