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1.
the space between the thighs of a woman close to the pubic region. if these thighs touch and cause persperation throughout the day, this area is potentially stinky and now called an onion patch.
look at that onion patch...i'd still hit that.

damn i went down on her after work and she had one nasty onion patch.
by joeltfoster January 25, 2010
 
2.
Someone who smells bad and/or is quite homosexual and/or visual unappealing.
"Codie is such an onionpatch"
by WizCorifa May 06, 2012