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Yet another example of the versatility of the word head --

It is (possible obsolete) slang for a drug user, especially hallucinogens. A quote from Stephen King and Peter Straub's The Talisman -- "They cranked the dolby because the heads liked it loud."

As a suffix -- pothead, acidhead, etc
Possibly from the Jefferson Airplane lyric "Feed your head"

As a prefix, the ever popular headshop, frequently found as part of a record store, where drug paraphanalia, body jewlery, incense, and other drug-culture related items can be purchased.
When in a modern headshop, it is forbidden to mention that you plan to use your purchases for illegal purposes.