Meaning of UNSTABLE in English

UNSTABLE

un ‧ sta ‧ ble AC /ʌnˈsteɪb ə l/ BrE AmE adjective

1 . likely to change suddenly and become worse ⇨ instability :

The political situation is still very unstable.

an unstable relationship

2 . something that is unstable is likely to move or fall

3 . someone who is unstable changes very suddenly so that you do not know how they will react or behave:

a mentally unstable man

4 . an unstable chemical is likely to separate into simpler substances

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THESAURUS

▪ mentally ill having an illness that affects your mind and your behaviour:

Many mentally ill people are treated in the community.

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He was declared mentally ill and unfit to stand trial.

▪ crazy informal mentally ill:

I couldn’t think straight. I felt like I was going crazy (=becoming crazy) .

▪ mad old-fashioned mentally ill. This word is now usually considered offensive, and is usually used in a different meaning, when you think that someone's ideas are not sensible:

Swift himself went mad (=became mad) later in life.

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a mad old woman

▪ insane [not before noun] old-fashioned having a serious and permanent mental illness:

She went insane after her two young sons were murdered.

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a hospital for the criminally insane

▪ disturbed not behaving in a normal way because of mental or emotional problems, especially problems that are caused by bad experiences:

She teaches emotionally disturbed children.

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Her experiences left her deeply disturbed.

▪ unstable having an emotional state that often changes very suddenly:

Her mother was mentally unstable.

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He lived in a small Putney flat with his ageing Aunt Bunny, and his emotionally unstable sister, Nancy.

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He was too unstable to be a leader.

▪ deranged behaving in a crazy or dangerous way, usually because of being mentally ill:

A deranged young woman entered the school and took the life of one young boy.

▪ psychopathic having a serious and permanent mental illness that causes violent or criminal behaviour:

The film is about a psychopathic killer.

▪ psychotic suffering from or caused by a serious mental illness that changes your character and makes you unable to behave in a normal way:

psychiatric services for chronic psychotic patients.

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There is a tendency for psychotic illnesses to be inherited.

▪ neurotic relating to or suffering from a mental illness that makes you unreasonably worried of frightened:

His mother was neurotic and insecure.

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She's neurotic about her weight.

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A bored or lonely horse may become so neurotic that it chases itself around in circles.

Longman Dictionary of Contemporary English.      Longman - Словарь современного английского языка.