The practice of slapping someone's face with your penis, usually when erect. This is often performed as a joke when the recipient is either asleep or otherwise incapacitated.
Pablo is very into cock slapping, so he tried it on his girlfriend while she was blowing him, and she liked it.
"For four, what the fuck happened to Clive Winston, you pricks?! In his place we've got some multicoloured, giggly J-pop creature, and you can't play classic rock with that...thing. It's like cock-slapping the Mona Lisa."
-Ben "Yahtzee" Croshaw in his Guitar Hero III review
A phrase or expression used to communicate that you think that somebody is lying or "pulling your leg." It basically means that you think somebody is feeding you a pile of bullshit.