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1.
A rare, genetic condition found in males, usually surfacing at the on-set of puberty; the result of which is a swift plummeting of the carrier's overall intelligence.
Young Abraham quickly went from smart as a monkey to dumb as a chimp, as a result of his dumbening.
by Michael T. Lynch Jr., Scientician October 12, 2003
 
2.
Describes the ever increasing lack of common sense and/or intelligence often displayed in society today.
This is an example of the dumbening.
"What language do they speak in England?"
"Did he really ask that?"
"Yeah, it's The Dumbening."
by Jus. July 16, 2008