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1.
Coined by Adam Carolla, an Uncle Dude is a guy who breaks the "guy code".

Can also be used when a guy betrays other guys by way of giving information detrimental to men to women.
Greg Behrendt is an uncle dude.
by Thug Bear January 11, 2009
 
2.
A guy that tells his wife/girlfriend about the exploits of his friends, thus getting his friends in trouble with their wives/girlfriends. Usually done preemptively to try to maintain innocence.
Jerry is such an uncle dude, he told his wife we went to a massage parlor and she told my wife.
by Owwwmyballs September 01, 2009