Entry in progress—B.P.
 
Wikipedia: Tin foil
Tinfoil or tin foil is a thin leaf made of tin.
 
Actual tin foil was superseded by cheaper and more durable aluminium foil after World War II, and aluminum foil is sometimes confused with “tin foil” because of its similarity to the former material.
 
Wikipedia: Tin foil hat
A tin foil hat is a piece of headgear made from one or more sheets of aluminium foil or similar material. Alternatively it may be a conventional hat lined with foil. Some people wear the hats in the belief that they act to shield the brain from such influences as electromagnetic fields, or against mind control and/or mind reading.
 
The concept of wearing a tin foil hat for protection from such threats has become a popular stereotype and term of derision; the phrase serves as a byword for paranoia and persecutory delusions, and is associated with conspiracy theorists.

The reasons for their use include the supposed prevention of perceived harassment from governments, spies or paranormal beings. These draw on the stereotypical images of mind control operating by ESP or technological means, like microwave radiation. The effectiveness of tin foil hats is disputable; however, the belief in their necessity is popularly associated with paranoia or mental illness.
 
Scientific basis
The notion that a tin foil hat can significantly reduce the intensity of incident radio frequency radiation on the wearer’s brain has some scientific validity, as the effect of strong radio waves has been documented for quite some time. A well constructed tin foil enclosure would approximate a Faraday cage, reducing the amount of (typically harmless) radiofrequency electromagnetic radiation inside. A common high school physics demonstration involves placing an AM radio on tin foil, and then covering the radio with a metal bucket. This leads to a noticeable reduction in signal strength. The efficiency of such an enclosure in blocking such radiation depends on the thickness of the tin foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimeter-thick tin foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked, although tin foil is not sold in this thickness, and numerous layers of tin foil would be required to sustain this effect.
 
The effectiveness of the tin foil hat as electromagnetic shielding for stopping radio waves is greatly reduced by the fact that it is not a complete enclosure. Placing an AM radio under a metal bucket without a conductive layer underneath demonstrates the relative ineffectiveness of such a setup. Indeed, because the effect of an ungrounded Faraday cage is to partially reflect the incident radiation, a radio wave that is incident on the inner surface of the hat (i.e., coming from underneath the hat-wearer) would be reflected and partially ‘focused’ towards the user’s brain. While tin foil hats may have originated in some understanding of the Faraday cage effect, the use of such a hat to attenuate radio waves belongs properly to the realm of pseudoscience.
   
Kossary - dKosopedia
Tin-Foil Hat
A device made of tin-foil worn on the head to ward off the voices and signals from three letter agency mind control devices and/or outer space. A label for outlandish conspiracy theorists derived from this headgear, which is sometimes actually worn at certain conventions as a joke or a signal that conspiracy debate is welcome. When used in discourse, it refers to stereotypical crazy conspiracy theory. It is beginning to come into use as a self-deprecating term to acknowledge that what the person is about to say sounds somewhat outlandish. Used similarly to asbestos long-johns. Often used as a deprecatory term to dismiss theories like Bush Knew or even Iran-Contra as if there were no evidence for these.
     
Urban Dictionary
tinfoil hat
June 30, 2005 Urban Word of the Day
1. A hat made from kitchen aluminum foil or other pliable metallic substance, with the supposed purpose of shielding the wearer’s brain from mind control/surveillance by various supernatural or conspirital organizations.
2. Pertaining to various supernatural or conspirital phenomena.
Mike is still talking about space aliens? He must have left his tinfoil hat at home.
by 1Spectre4U Aug 12, 2003
   
Urban Dictionary
Tin Foil Hat
Tin Foil Hat, also tinfoil hat, or tfh, a general term for a piece of headgear made from one or more sheets of tin foil, aluminium foil, or other similar material. Some people wear the hats in the belief that they act to shield the brain from such influences as electromagnetic fields, or against mind control and/or mind reading. The concept has become a popular stereotype and term of derision; in Internet culture, the phrase serves as a byword for paranoia.
that guy is so paraniod he needs a tin foil hat.
by bkdouble Oct 5, 2006
   
Urban Dictionary
tinfoiler
someone so ***out there***, that they wear tinfoil hats—or perceive messages from their amalgam tooth fillings. its roots are from mental institution paranoia and has progressed to today’s contemporary society.

Politically, this may be a derogatory comment towards conspiracy theorists and is akin to birthers and their ilk.
He’s a real tinfoiler: he believes those stories that other ethnicities eat fetus baby soup—just because it is on the ‘Net.

She thinks the ants took her sunglasses from the picnic—she’s a tinfoiler.
 
Tinfoilers believe that the End of Days is nigh, because of who won the election.crazy people conspiracy theorists baby soup birthers mental illness
by Mumsie Dogma Oct 11, 2009
   
Google Groups: alt.conspiracy
Newsgroups: alt.current-events.clinton.whitewater, alt.conspiracy
From: .(JavaScript must be enabled to view this email address) (Bill Kasper)
Date: 2000/02/29
Subject: Re: More on Project Echelon!!
 
=maybe you’re one of the mother fuckers that works for the NSA or CSE
 
See Robertson?
 
It is just transparently obvious.
 
Even to this ee cummings-style tin foiler.
 
the Greenspun family server
As some of you may also know however, when the original publication date of this book was announced, I was flooded with emails from people who were concerned that my book would make them look like tin-foil hat” lunatics.
(...)
—FM (.(JavaScript must be enabled to view this email address)), April 11, 2000
         
Google Groups: us.military.army
Newsgroups: us.military.army
From: Colin Campbell
Date: 2000/07/20
Subject: Re: A Question for Combat Arms Soldiers
 
The tinfoil hat brigade strikes again.
 
Google Groups: comp.software.year-2000
Newsgroups: comp.software.year-2000
From: .(JavaScript must be enabled to view this email address) (Jacques Bernier)
Date: Mon, 07 May 2001 08:29:21 GMT
Local: Mon, May 7 2001 2:29 am
Subject: Re: Underpinnings of Irrationality
 
Napoleonic complex come to mind but he swears he’s the King of England. Multiple personality (Goobers-DocDwarf-King-Tinfoiler).
   
Google Groups: rec.arts.sf.fandom
Newsgroups: rec.arts.sf.fandom
Followup-To: rec.arts.sf.fandom
From: .(JavaScript must be enabled to view this email address) (Avedon Carol)
Date: Fri, 09 Nov 2001 14:50:00 +0000
Subject: Re: The Afghan Oil Pipeline (was: Something else
 
>On Tue, 30 Oct 2001 23:41:05 +0000, Robert Sneddon
> wrote:
>> Some of the tinfoil hat brigade in alt.conspiracy are claiming the
>>Afghan war is a pretext to take over the country and run an oil pipeline
>>from the Urals down through Afghanistan and into Pakistan. They have
>>derived some rather tenuous links between Bush oil concerns, Dick Cheney
>>and a nebulous Russian oil Mafia.
(...)
I think the Spinsanity discussion is well-reasoned, which is why I also think it’s a good idea to keep an eye on “the tinfoil hat brigade”, who they are, and what they are saying. 
 
Google Groups: or.politics
Newsgroups: or.politics
From: “Dave Thompson”
Date: Sat, 10 Nov 2001 16:02:40 -0800
Local: Sat, Nov 10 2001 6:02 pm
Subject: Attacks from the tinfoil hat brigade
   
Google Groups: alt.politics.democrats.d
Newsgroups: alt.current-events.clinton.whitewater, alt.politics.democrats.d, alt.politics.liberalism, alt.politics.republicans, alt.society.conservatism, alt.society.liberalism
From: “Dana”
Date: Sat, 22 Dec 2001 17:23:44 -0900
Local: Sat, Dec 22 2001 8:23 pm
Subject: Ensuring Airport Insecurity
 
“As a former employee of a defense department contractor, I’m fairly certain that three of these agencies are the CIA, the NSA and the Tonopah Test Range - better known to tin foilers here as Area 51”.
   
Google Books: alt.fan.cecil-adams
Newsgroups: alt.fan.cecil-adams
From: JmG
Date: Wed, 13 Mar 2002 23:51:06 -0500
Local: Wed, Mar 13 2002 10:51 pm
Subject: Re: Tinfoil hat time…
 
The AM radio tinfoil hat brigade has blamed Bill Clinton for the attacks but their obsession with the past President borders on mental retardation.
 
Google Groups: alt.fan.cecil-adams
Newsgroups: alt.fan.cecil-adams
From: Greg Goss

Date: Sun, 03 Aug 2003 00:53:17 GMT
Local: Sat, Aug 2 2003 6:53 pm
Subject: Re: Two more Republicans in desperate need of lives
 
There were a bunch of tinfoilers discussing the WTC “conspiracy”.
   
Tech SUpport Guy Forums   
iltos
11-Mar-2005, 01:05 PM
being a proud member of the loyal opposition, or, as you call it, mulder, the tin foil hat brigade, i was a bit stunned to recieve this qualification…..