Originally Posted by Get Smart
doesnt semi-formal mean "wear a suit"? I def wouldnt wear a tux to "semi formal"
It's basically archaic usage now, but yeah, semi-formal traditionally meant "black tie." To this day, invitations you get from by-the-book operations (e.g., embassies) will say "business attire" or "lounge suit" for a suit and "Semi formal" or "Black tie" for a dinner jacket. "Formal" in the evening is white tie and tails, which is really rare.