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A reference to a woman's backside - specifically a really well-formed one. So called because, as we all know, crossing a mule and an onion results in "an ass that'll bring tears to your eyes".
"Damn, look at the mule onion on that one!"
by Heathen 2.0 April 28, 2008

Words related to Mule Onion

ass backside booty mule onion