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1.
A character exhibited by a person, what many consider as a stage of mental retardation, until they give a little deeper thought to discover those actions have witty logic and humane reason.
Tom was ridiculed by his classmates until they discovered his gumpism
by rash driver July 11, 2010
 
2.
Mental retardation to an extreme degree.
I forgot the name of the president as a result of my gumpism.
by T-Rexus January 14, 2006