Meaning of “unstable” - Learner’s Dictionary

unstable

adjective uk us /ʌnˈsteɪbl/
CHANGE

likely to change or end suddenly:

an unstable situation
an unstable economy
PERSON

If someone is unstable, their moods and behaviour change suddenly, especially because they are mentally ill.

MOVE

not fixed or safe and likely to move:

That chair looks a bit unstable.

(Definition of “unstable” from the Cambridge Learner’s Dictionary © Cambridge University Press)