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The belief that sweatshirts or hoodies should be worn at ALL times. Females who practice this are usually considered tomboys, skaters, insecure, etc. Although some cases actually are, most believers are simply avid lovers of sweatshirts.
Boy: It's so hot outside! Why on earth are you wearing a sweatshirt?

Girl: Haven't you ever heard of sweatshirtism?
by laumm. January 12, 2011