|Some dare call it|
The tinfoil hat is a sort of mocking symbol of paranoia and conspiracy theory advocacy, derived by the use of some paranoid schizophrenics with persecution and mind control delusions of metal foil hats to protect them from perceived mind control rays (no, seriously, but do have some sympathy, unless they get into office). Despite the name "tinfoil", the hats are almost universally made from aluminum in the Americas or aluminium in the rest of the world.
The term came about because of serious suggestions by true believers that constructing a simple hat out of aluminum foil could protect their brains from being bombarded with mind control rays or whatever it was they were paranoid about (essentially, it is a very lame interpretation of how a Faraday Cage[wp] works). The idea was so laughable that it quickly took on a life of its own as a sarcastic way of referring to any especially preposterous notion that usually involves a mixture of paranoia with pseudoscience.
 Advanced scientifical forms
Some claim that copper mesh works better. Personally, we prefer a zinc-manganese alloy in a bilayered hexagonal array.
In 2005 a group of engineering and media students at MIT performed attenuation versus frequency measurements for different tinfoil hat designs, concluding that the hats actually increased the intensity of RF energy at certain frequencies, with the implication that the tinfoil hat fad was actually a government plot to increase the effectiveness of mind control rays.  Notably the specific frequencies amplified exist in the spectrum of the emission band owned by the government! Of course the government could have blackmailed the MIT students to write that false report to make you take off your tinfoil so they can affect you with mind control rays. Then again the government could also be blackmailing me to write this so you will keep your tinfoil cap on to better amplify the mind control rays.
You should probably take you tinfoil hat off now. Or put it on if you don't have it already. (Wearing the hat only when you want it off and removing it only when you want it on confuses the government mind readers as to your true intentions). Maybe you should make one of better construction, but then you would have to learn about electromagnetic waves, but all educational materials may or may not be instruments of government deceit. But, no! That's what they want you to do, so you can't hear them sneaking up on you with their silent black helicopters to carry you to Area 51 and strip you of your valuable bones.
Step by step instructions for making one are available.
 See also
Other terms that have taken on a life of their own in the same way, for similar reasons, include:
- The Atlantic: Tin Foil Hats Actually Make it Easier for the Government to Track Your Thoughts 28 September 2012 — The Atlantic ruining the final resort of conspiracy theorists everywhere.
- Michael Menkin's tutorial on how to build "thought screen helmets" that stop "space aliens from abducting humans" (apparently not a poe). From a satisfied customer: "Since trying Michael Menkin's Helmet, I have not been bothered by alien mind control. Now my thoughts are my own. I have achieved meaningful work and am contributing to society." You see?
- Do tinfoil helmets provide adequate protection against mind control rays?, at The Straight Dope