tin-foil hat

Noun

tinfoil hat (plural tinfoil hats)

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.
   • Jeanne Due's ingenuity has sent the most remarkable of hats, this being the now notorious tinfoil hat, pictured elsewhere. The strangest part about this weird creation, however, is the fact that it is not unbecoming, but quite wearable.1916, Roger W Allen, editor, Millinery Trade Review, volume 41, page 27

   Synonyms:

   TFH

2. (derogatory, metonymically) A conspiracy theorist.