It means:
* Your words are more poisonous and harmful than the venomous snakes of the Nile river.
Explanation:
* "Thy tongue": Refers to the person being addressed (Roderigo)
* "Outvenoms": Means "surpasses in poisonousness"
* "All the worms of Nile": Refers to the venomous snakes found in the Nile river, known for their deadly bites.
Iago is using this hyperbole to emphasize how venomous and destructive Roderigo's words are. Iago is manipulating Roderigo, encouraging him to speak negatively about Othello, and is using this line to emphasize the danger of spreading rumors and lies.
It's important to note that the word "worm" is an archaic term for a snake.
In this context, Iago is saying that Roderigo's words are more dangerous than the most poisonous snakes in the world.