The nostalgic feeling people experience after the end of the 2024 Olympic Games.

A French neologism: JO = Jeux Olympiques (Olympic Games) + nostalgie (nostalgia).

Source: https://rmcsport.bfmtv.com/jeux-olympiques/est-ce-qu-il-y-a-un-numero-vert-la-j-ostalgie-plane-sur-la-france-apres-la-fin-des-j0-2024_AV-202408120230.html