Be one card short of a full deck phrase informal
To be unintelligent or mentally deranged.
She was one card short of a full deck and always thought everyone was trying to kill her.
Nick said that he liked standing under a tree in a thunderstorm. I think he may be one card short of a full deck.
He was one card short of a full deck, but he was industrious.
Extremely stupid
Very stupid, or silly
To be extremely stupid; not to be alert
(n): a fool person,
(adj): stupid, foolish, silly
The verb "be" should be conjugated according to its tense.