"You know who you are, u prat."
That made me look up "prat." It's English slang, going back to the 1500s, meaning "a buttock." Later, it came to mean both buttocks, i.e., a "bottom." (Source: OED.)
"Why, she's getting groggy on her pins, and if you don't pipe rumbo, she'll go prat over nut." That's from 1846.
By 1955, it was slang for "An idiot, a fool; an ineffectual or contemptible person." Joe Orton used it in 1964 in "Entertaining Mr Sloane": "Go on, you superannuated old prat!"
I must say that before looking it up, I pictured it as a fish. I was thinking: sprat.
ADDED: This planted it in my head that Brits insult each other with fish:
IN THE COMMENTS: Laslo Spatula said: "I'm surprised Althouse din't also link o this Monty Python fish skit:"
The answer (to quell your surprise (I'm always alarmed at what surprises people around here)) is that — although I've had the DVD on my shelf for years — I've never gotten very far into "Monty Python's The Meaning of Life." There's only so much of this sort of foolery I can take in one sitting. A half hour is best for comedy. In the early days of movies, that was well understood. For example, this half hour of Chaplin from 1918:
That's all you need and all you want.