Xixox

I've always said it as "Suck a fart outta my ass." It rolls off the tongue nicely when done with a new yorker accent.

My latest has been 'douchenozzle'. For work-safety I'll fall back on 'chowderhead'

For general descriptions...
... dumb as a sack of hammers
... sharp as a sack of wet mice
... pleasant as a tax audit
... about as smart as that {whatever inanimate object happens to be handy}
... breath like a sack of assholes

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Incompetence
When you earnestly believe you can compensate for a lack of skill by doubling your efforts, there's no end to what you can't do.