I prefer... not the sharpest tool in the shed.
or whack job, or nut bar, screwball, screw loose.
Loose nut behind the wheel. ( we use that here at work, when customers complain about their cars,)
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Rhonda, Bailey, Josie and my angel Bella Rue'
"Until one has loved an animal, a part of one's soul remains unawakened."
~Anatole France~