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1.
The mental effect of realizing that a basic assumption of your entire life was wrong from day one, usually resulting from a totally new thought, comment, or experience that flies in the face of everything you currently know.

See also; mindfuck.
Mental whiplash is the psychological equivalent maxing out your revs in Third, and then accidentally shifting into Reverse instead of High.
by Downstrike October 22, 2006
 
2.
A serious case of mental confusion that arises when a fashionista decides to wear normal clothes for once, thus making normal clothes wearer's minds go crazy.
Whoa! I just saw Mary-Kate and Ashley Olsen in normal clothes and experienced mental whiplash!
by Yuppthat'sme November 06, 2011
 
3.
The feeling one gets after a painful sudden realization that something they once thought is not true.
"I had total mental whiplash when I found out it was Monday. I had thought it was Friday all day!"

"I thought I saw Susie at the mall so I gave her a hug, but she turned around and it turned out that it was just some random dude. Absolute mental whiplash."
by CrescentSunGoddess March 05, 2009