Pronunciation: Brit. /ˌtɪnfɔɪl ˈhat/ , U.S. /ˈˌtɪnˌfɔɪl ˈhæt/
orig. and chiefly U.S.
A hat made from tinfoil.
With allusion to the belief that such a hat protects the wearer from mind control, surveillance, or similar types of threat. Frequently either extraterrestrials or the government are imagined to be the source of such threats.
(via Oxford English Dictionary.)
The earliest cited use of the term in the OED is from 1982, though the concept has been around much longer – since a 1927 science fiction story, according to the Wikipedia entry.
Be sure to check out The Tinfoil Hat Song. If that speaks to you, you might want to get Tinfoil Hat Linux. You might also want to read “MIT Researchers Claim Tin-Foil Hats Don’t Stop Government Mind Control Rays.”
Myself, I’ll stick to my tinfoil-lined duct tape wallet for the time-being. Aliens and men in black may be scary, but ID snafflers seem like a more real and present danger. How To Make A RFID Blocking Wallet.