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1.
a lady, or in this age, gentleman, who is selectively promiscuous.

first seen in Ireland in the 1970s
She must have yo-yo knickers to have slept with that many guys.
by heather September 24, 2004
 
2.
An Easy Girl. a loose Woman. A slag
"I shagged That Girl last night"
"What Yo Yo Knickers? Who hasn't?"
by SteveDave May 09, 2003