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Interesting question, and I can't really answer it, not being American - "prat" basically means an idiot, but it's quite a useful word because it conveys a real sense of contempt (but often more for the opinion or act than the person responsible for it, in a weird way), and can easily be intensified as in this instance, without actually being vulgar. You can also use it as a verb, eg. "Stop pratting around on Hacker News and do some work!"

I also feel like it was 100% the correct word to use in this instance!

It's a soft version of "idiot". So maybe an affectionately spoken "doofus"?

If you google it, you'll see it's synonymous with "idiot".