When a male or female takes part in sexual activity within a group of friends but does so in such a way that no one notices because of clever timing and casual body language.
"Where do you think Rob went while he strolled off in the park earlier?"
"He was probably fucking Rosie up that tree"
"Ahhhhh, Smart Casual"
"Why did it take the police so long to track down the rapist?"
"Because he's a Smart Casual rapist, get me?"