{not} [play] with a full deck

to lack intelligence or rationality

The idiom "not play with a full deck" likely originates from the world of card games, where a complete deck is necessary for fair and functional play. In this context, someone "not playing with a full deck" suggests a lack of mental completeness or sharpness, akin to a deck of cards missing some cards. The expression is used figuratively to describe individuals perceived as mentally challenged, irrational, or lacking common sense.

After his strange explanation for the missing files, it's clear he's not playing with a full deck.
I wouldn't take financial advice from him; he seems like he's not playing with a full deck.
She insists that her bizarre theories are true, but everyone thinks she's not playing with a full deck.
Considering his recent decisions, some are starting to believe that the politician may not be playing with a full deck.
The boss questioned his employee's ability to handle complex tasks, suggesting he might not be playing with a full deck.