verb/pʁɑ̃.dʁə de vɛ.si puʁ de lɑ̃.tɛʁn/
to be deluded, to be very naive, credulous and/or gullible, to be sorely mistaken, to have another think coming, to think the Moon is made of green cheese
Synonyms: se faire des films, se faire des idées, se mettre le doigt dans l'œil
Literally, “mistake bladders for lanterns”.