away with the fairies

Someone who is away with the fairies is in such a dreamy state that they are not totally in touch with reality and give the impression of being slightly mad.
“It’s no use trying to explain the problem to her – she’s away with the fairies!”

go bananas

If someone becomes very emotional and starts behaving in a crazy way, they go bananas.
“If you announce that you are going to drop out of school, your parents will go bananas!”

(a) basket case

A person whose agitated mental state leaves them helpless or unable to cope with things is called a basket case.
“Jenny will turn into a basket case if this stressful situation continues.”

bats in the belfry

If you say that somebody has bats in the belfry, you mean that they are eccentric or crazy.
“He comes up with the craziest ideas – he must have bats in the belfry!”

have you gone out of your mind?

If you ask someone if they have gone out of their mind, you think they are crazy, foolish or insane.
“You’re going to ask for a 100% increase in salary? Have you gone out of your mind?

the lights are on, but no one is home

This is a humorous way of referring to someone who is lacking intelligence or sanity, or to someone who is simply preoccupied and not paying attention.
“Forget about Andy today – the lights are on but no one is home!”

lose one’s marbles

If someone loses their marbles, they become mentally confused, or no longer behave sensibly or rationally.
“The old man is acting very strangely. He seems to have lost his marbles.”

not all there

Someone who is not all there behaves strangely at times, or seems a bit odd.
“Our new neighbour wears a hat and coat even in summer; he’s not all there!”

not in one’s right mind

Someone who is not in their right mind is not behaving in a logical or sensible way.
“Julie wasn’t in her right mind the day she bought that outfit. It doesn’t suit her at all!”

not playing with a full deck (of cards)

Someone who is not playing with a full deck (of cards) lacks intelligence or does not have full mental abilities.
“Old Mrs.Whitehead was not playing with a full deck when she bought that fancy lawnmower!”

go nuts

To say that a person has gone nuts means that they have become completely foolish, eccentric or mad.
“I think the old lady has gone nuts! It’s very hot today and she’s wearing a fur coat!”

(as) nutty as a fruitcake

Someone who is (as) nutty as a fruitcake is insane or crazy.
“Don’t pay attention to what the old man says; he’s as nutty as a fruitcake!”

off your rocker!

If you tell someone that they are off their rocker, you think they are completely crazy.
“You’re going to give all your money away? You’re off your rocker!”

(have) rats in the attic

If you say that someone has rats in the attic, you mean that they are a bit mad or that their behaviour is eccentric.
“She keeps repeating the same question. I think she’s got rats in the attic.”

(have) a screw loose

If someone has a screw loose, their behaviour is strange and they appear slightly mad.
“She wears a fur coat in the summer – she must have a screw loose!”

stark raving mad

To say that someone is stark raving mad means that they are completely crazy.
“You’re stark raving mad to go sailing in this weather.”

stir crazy

If a person goes stir-crazy, they become very agitated or nervous because they have been confined to a place for too long.
“After several days in quarantine, people were going stir-crazy.”

take leave of one’s senses

To say to someone ‘have you taken leave of your senses?’ means that you think their behaviour is crazy.
“You’re going skiing in this blizzard? Have you taken leave of your senses?”