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1.
A relative or other pseudo-family member, not necessarily limited to uncles, who can always be counted on to say inappropriate things, express unpopular opinions, or otherwise make everyone uncomfortable at family gatherings.
"Dude, who is that guy!?" "Don't mind him, he's just my Uncle Asshole."

"You think you're Uncle Asshole is bad? MINE voted for Santorum!"
by Caroline Noire March 01, 2012