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1.
1) The belief that body weight, size or type accounts for differences in human character or ability and that a particular body weight, size or type is superior to others.

2) Discrimination or prejudice based on body weight, size or type.
Sizist people are so threatened by fat civil rights activist, they make a point to insult fat people whenever they can, i.e. at the bottom of the previous definition.
by kbee March 30, 2005
 
2.
hate on cute little short girls for being small and cute.
HEY THATS SIZIST!"...."I'm real sizist and make fun of short girls.
by short girl. June 20, 2011