1. A conjunctive phrase used to introduce a reason that strengthens or reinforces a previous statement, indicating that something is even more true or valid given the additional circumstance
He should have known better, all the more so because he had been warned repeatedly.
Ele deveria ter sabido melhor, tanto mais que tinha sido avisado repetidamente.
2. Used to emphasize that a statement becomes more relevant or important given an additional factor or reason
We must act now, especially since the deadline is approaching.
Devemos agir agora, tanto mais que o prazo está se aproximando.