n. English term, primarily used in United Kingdom. The literal meaning is "bottom" or "rump"; aka backside, buttocks, sacrum, tail end. This lends itself to the slang meaning of "ass," or "clueless person of arrogant stupidity." It is not always directly translatable to American slang. For example, if you used the term "prat hat" in the U.K., you would likely be laughed out of town by the locals.
I can't believe what an overbearing idiot he is. What a prat!