تعریف و معنای "{not} [play] with a full deck"

What is the origin of the idiom "not play with a full deck" and when to use it?

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.