Unprovoked crippling shooting arse pain you get every now and then (once in a 'blue moon') . Feels like someone is stabbing you in the arse hole for 10 seconds.
Lucy: Arrgghhh!!
Becky: What mate?!
Lucy: feels like someone just stuck a javelin up my butt!
Becky: That'll be a blue moon mate
A sexual act in which blue slurpee, soft drink or food dye is placed into a menstruating vagina. The subsequent period blood which comes forth, along with the blue drink is known as the blue moon, due to the blue colour and the period happening once a month, or moon.
Steve: Shana told me she was on her period last night, so I decided to give her a blue moon.
Jay: Cool digs man.