Passé describes something that's out of style or "so yesterday!" What your parents think is cool is usually very passé, but sometimes their old stuff is stylish again, like the Lynyrd Skynyrd T-shirt you borrow from your mom.
From the French passer, "to pass," passé came into English use in the 18th century, and its accented "é" is still used in the 21st century. Pronounced "pass-AY," the adjective passé is a classier way to call something outdated, unfashionable, and so out of touch with trends. Your floppy pop-star haircut may be passé if a crew-cut band becomes all the rage, but even saying "all the rage" is itself probably passé. If it's current and in style, it's not passé.