Lacking intelligence or common sense.
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Not thinking clearly, or short on good sense.
When someone is not playing with a full deck, they act in ways that seem silly or out of touch.
It's an informal, slightly teasing way to say someone isn't all there.
"Off Your Rocker"
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Acting in a wild, irrational way — stronger and more dramatic
"Out of Your Mind"
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Doing something extremely reckless or surprising, not about judgment
"Not Playing with a Full Deck"
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Lacking full mental sharpness or the ability to make good decisions