tinfoil hat

noun2

1. •
   A piece of headgear made from one or more sheets of tin foil, aluminium foil, or other similar material; especially when worn in the belief that it shields the brain from electromagnetic fields or against mind control or mind reading.

   • To prevent them from reading his mind, he wore a tinfoil hat at all times. — 2001
2. •
   (disapproving) A conspiracy theorist.