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.
Daughter: So I really want all my friends to have a great time. I was thinking it would be a good idea to have some candy out on the tables for people to snack on.
Mother: Well, if you want them to really enjoy themselves, candy is dandy but liquor is quicker.