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1.
Any half-hearted and disingenuous attempt by the mainstream to pass itself off as new, nonconformist, or anti-establishment.

vb. mainstream ("normal", trendy, majority, conformist, establishment) + steam (to let off steam or relieve pressure)
"GAP is donating half of the sales on red items to support Women's Rights in the Third World!"
"They're just mainsteaming. don't believe that crap."
...
"I went to a hippie festival last month to check out the scene, but I realized I was just mainsteaming. I don't actually understand their deal."
by HonourGuard March 19, 2007
 
2.
A deviation of the word "Mainsteam" when it comes to the faking of orgasms.
"Because its so MAINSTEAM to fake"
by Not Joel, It's true! July 27, 2006