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1.
Someone who is anti - mainstream has a strong dislike of trendy things and people. Many subcultures have preached about being anti - mainstream, most notably punk and heavy metal. Death metal, Black metal, and a few lesser known subgenres of punk rock remain the longest standing anti - mainstream subcultures while others have either become popular (I.e punk) or merged into other more popular subcultures(I.e classic metal and thrash metal). While music subcultures have the biggest anti - mainstream message other subcultures based around religion (I.e LaVeyan Satanism), politics(i.e non partisans), and even literature have emerged. Most trendy things start out anti - mainstream and become popular usually after a slight change in image (I.e in the 1920's united states when white people began playing jazz it became acceptable).
Punk was the first major anti - mainstream subculture, shortly followed by heavy metal and hip - hop.
by Thatheadbanger July 25, 2009
 
2.
when you won't listen to a perfectly good song just because everyone and their grandmother and their dog and mtv plays it.
jebediah "i was going to listen to kanye west but then i realised how gay he is."

sally "you're just anti main stream you ass pirate."
by ethanbockcallsmary May 14, 2010