The term tin foil hat is generally used as a derogatory remark toward someone advancing a conspiracy theory or other story that is considered unbelievable. It appears to have been coined to describe people who claimed UFO sightings or claimed to have been previously abducted by aliens. It was suggested that such a person might wear a hat made of tin foil in order to keep aliens from intercepting his or her brainwaves. There are some people who actually construct such hats, however, believing they will protect against mind control programs, radio waves, and electromagnetic radiation (EMR).
In fact, a type of device made from aluminum or tin foil may help reduce EMR. It isn’t foolproof, since hats or other protective gear aren't solid structures. Electromagnetic radiation is also said not to be harmful, despite earlier possible concerns.
Some people also say that those who promote conspiracy theories or government conspiracies are part of a tin foil hat brigade, or a tin foil hat group. Those alleged to be a part of such a group may or may not believe that a hat of tin foil is useful. Anyone that actually does wear such a hat is usually considered to be mentally disturbed. The term is most often used to ridicule people, even those who would never consider wearing accessories made from foil. The wilder the conspiracy theory, the more likely the term is to be used.
Many times, when debates arise over various conspiracy theories, comments referring to tin foil hats are used to discredit the person attempting to advance some cover up or other questionable act by a government. On political message boards people say things like, “I see you’re wearing your tin foil hat,” to show that they think the theory has no basis and that the person suggesting it is simply paranoid. It can also be used in a self-deprecating way such as in saying, “I know you’ll think I’m wearing a tin foil hat, but I believe this conspiracy is real.”
A tin foil hat may also be known as an aluminum foil helmet. Studies have been done to test their effectiveness, although such studies are often not given much credibility. There are also people that sell hats made from this material, and many are available in joke stores and online. It might just make a good gag gift if given in the spirit of fun.