When a man and a woman are fornicating and the man sticks a zuccini up her bum. He cuts off the other half, puts it in his mouth, and sticks it in the frontal region. While eating her out she has to shoot both of them out simultaneously, and try to get one half back into his mouth. For this to be a proper 'Welsh Zuccini' the man, or woman, must be of Welsh origin.
"Dude my wife got a double Welsh Zuccini in my mouth last night!"