When you are having unprotected vaginal sex from behind and when you pull out and climax, you take a couple steps back, then run up behind the female and slap her on the ass with your jizz covered penis.
Dude, this chick was soooo pissed when I gave her the Iowan Fishtail!
The use of the word "blue" to refer to sexually explicit content was first recorded in Scotland in 1824. "Blue" meant "lewd" because prostitutes dressed in blue gowns.