AMEN
Is an akronism for the Hebrew words Emet, Melech, Nachman.
True King (who is) Merciful.
It's not Latin. It's as old as the Prophets.
It's solidly a Rabbinic thing. It's something everyone does/says. But its from Chazal. The Sages of Israel. You know it's right, too.
In fact, we should all say it 3 times… when we really mean/feel it. Like after a prayer for our POTUS. Or, for our country, the miserable people anywhere/everywhere. Amen Amen Amen.