throwing or shooting something (dice, pong ball) under the leg. A form of showing off, or praying that somehow your cockiness pays off in the luck department.
The Irish/UK version of Jesus H. Christ in a Chicken Basket, this phrase is used in extreme situations of shock, surprise, annoyance, or disgust. The addition of a well placed f-bomb can, of course, up the ante considerably.
Helpful chap in bathroom: "I say, it appears that your penis is dripping a rather greenish looking slime."
You: "What?! JESUS H. FUCKING CHRIST ON A BIKE!!"