Top Definition
Younger Brother Fat Syndrome. This syndrome affects younger brothers, especially youngest brothers in a family with all boy siblings, that are significantly heavier than their older brothers.
Did you see Johnny's younger brother waddling out of McDonald's? He's Twice the size of Johnny! What a case of Y.B.F.S.

Did you see Andrew's brother with YBFS fall down? He's so heavy he couldn't even get back up! He just rolled on his back like a turtle until some fire fighters came and helped to lift him back up!
by Dr. STOP YBFS February 18, 2011

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