Context: Pyrrhus, the Greek king of Epirus invaded the Italian peninsula in 280 BC. The campaign lasted five years, with Pyrrhus being victorious in several battles against Rome. However, each victory also resulted in casualties on his end as well.
After the second battle of the war, Pyrrhus is recorded to have said, "If we are victorious in one more battle with the Romans, we shall be utterly ruined." He still went on to defeat Romans several more times despite heavy losses.
This is how the term "Pyrrhic victory" came to be. Meaning a battle where the victor wins but incurs such significant losses that it's as good as a defeat.
The main advantage Romans had was that they could raise an army quickly to replenish their losses while Pyrrhus could not.