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When one 'fingers' a girl whilst she's on her period, then when she's close to orgasm, slap her in the face, leaving The Mark of the Red Baron.
"I went at it with this bird last night"

"oh yeah?"
"Yeah she was on though"
"Ah dude"
"Nah it's alright, I gave her the slap of the red baron!"
by PoetForSale March 22, 2010