Ergo means therefore. It is Latin that English borrowed and never quite made into ordinary speech. You meet it in old logic, in jokes that pretend to be proofs, and in the famous line Cogito, ergo sum — “I think, therefore I am.” In English it is a knowing, slightly showing-off “so…” People use it when they want the next sentence to feel like a conclusion, often with a smile. It is not the word for a real argument in a school essay; so, therefore, and that’s why are clearer. And it is not the ergo- that means “work” in ergonomic. That look-alike is a different Greek piece. This ergo is only “therefore.”