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1.
A term used to define a sexual practice wherein a female stands, legs-apart, above a male partner laying beneath her and proceeds to urinate and then defecate on top of him in said manner: "piddle in the middle, plop on top".

"No, seriously. Kendra was so annoyed that Shaatie tried to 'Cleavland Steam' her the other night, that she busted out a 'Barrel over the Falls' on his ass!"
by FallsGirl April 12, 2009
 
2.
A sexual routine that involves a female partner having her male partner lay down, while she stands above his chest, stomach, or pubic region (depending on her personal preference) and proceeds to pee, and then poo on top of him.
A variation of this practice is known as the "Barrel over the Falls ends in Tragedy", wherein the female will end the session by glopping a big smear of period blood on the male's face, however, this portion of the practice is reserved for occasions when the female is experiencing the peak of her monthly menstrual flow, hence allowing for good glopping action to take place.

The entire routine can be summed up in rhyme, as follows: "Piddle in the middle, Plop on top, Stain on brain", or alternatively, "Piddle in the middle, Plop on top, Glop on mop", and is widely considered to be the female equivalent of the Cleavland Steamer, Angry Pirate, and Donkey Punch routines.
When Jill was on the second day of her period, she knew it was the perfect time to give Jack a Barrel over the Falls ends in Tragedy, so she waited until nightfall and then let the ripest poo she could muster fall out of her bum and onto his chest, all the while demanding that he scream out in ecstasy.
by FallsGirlTheNextGeneration April 13, 2009