Fig.: to say or do something crazy, ridiculous, or stupid. To not be very bright

You really have rocks in your head: the leaning Tower of Pisa was not imported from Taiwan!

Ma hai proprio le pigne secche in testa! La Torre di Pisa non è stata importata da Formosa!

Driving 200 miles one way and 200 miles back just to go have coffee with your Italian friend in Miami? Do you have rocks in your head?

Guidare 200 miglia all'andata e 200 miglia al ritorno solo per andare a bere un caffè con la tua amica italiana a Miami? Ma devi avere le pigne in testa!

This slangy term, which originated around 1940, is frequently used in a snappy reaction to an odd statement or behavior. It probably implies that the person at issue has rocks for brains