In French, an entrée is what comes just before the main course. The word, just like hors d’oeuvre, was borrowed from music. Entrée is the first suite of music, or the intro piece. American’s once used this word correctly to mean an appetizer also. But this was back before WWI when it was common for upper class Americans to eat a multi-course meal comprised of soup, an entrée, a main meal and dessert.
During the war, rationing made it difficult for these Americans to follow this regiment and so the word entrée began to mean the main course. Perhaps they were being sarcastic by calling their main course the ‘entrée’, or maybe they really though of what they were eating as their entrée. Who knows? But we do know that we use it in this way still today.