Not playing with a full deck

Definitions

To imply someone is mentally unstable or lacking intelligence
To describe someone who is behaving irrationally or making foolish decisions due to a perceived lack of mental capacity. Can also be used to insult someone's intelligence.
To suggest someone is not in control of their actions or decisions
To imply that someone is not capable of making sound judgments or decisions, often due to a mental or emotional disorder. Can also be used to criticize someone's decision making abilities.
To indicate someone is not being truthful or honest
To imply that someone is lying or withholding important information, possibly due to a lack of mental clarity. Can also be used to question the credibility of someone's statements.

Sarah's brother always forgets important dates and loses his keys frequently. We joke that he's not playing with a full deck.

This idiom means that someone is not very intelligent or forgetful, as if they are missing some important mental faculties, like cards in a deck. It's often used humorously to describe someone's forgetfulness or lack of common sense. In this example, Sarah's brother is being teased for his forgetfulness.