Green’s Dictionary of Slang

1. a user of hallucinogens.

1967

Time 7 Apr. 60: Youthful mind-benders have tripped (or thought they did) on everything from airplane glue to morning-glory seeds, from nutmeg to black tea.

2. (also mind-expander) a psychedelic or psychotropic drug.

1967		Science News 22 July 80: Mescaline, the cactus-derived mind-bender.
1970		Times 26 Mar. 7: L.S.D., mescaline and other so-called mind-expanders.
1971		Asbury Park Press (NJ) 9 May 37/3: Pep pills, mindbenders exploded on the children of the ’50s.

3. (also mind-bend, mind-snapper) anything that, through the difficulty of its solution or comprehension, fig. ‘bends the mind’.

1965		New Scientist 18 Mar. 694: The other 86 mindbenders.
1969		N. Cohn Awopbop. (1970) 238: An epic three-hour freak-out every Saturday night, a veritable mind-snapper.
1971		Harper’s Mag. Dec. 97: What the Republican mind-benders are thinking about is individualized communication.
1981		J. Sullivan ‘The Russians are Coming’ Only Fools and Horses [TV script] Yeah, that’s a real mindbender Del that!
1993		K. Scott Monster (1994) 104: Gang members who are combat soldiers are subject to the same mind-bend as are veterans of foreign wars.