Numpty
A word that apparently is Scottish, but you use it by calling people that are idiots "idiots" without them realizing it because... well, they are idiots. Luckily, great minds think alike, so both me and the Scottish know what is up.
"Dude, Jim is such a fucking numpty, he keeps eating shrimp whole, he doesn't take off the carapace before he shoves it into his fat little mouth."
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