not solid and firm and therefore not strong, safe, or likely to last

That chair looks unstable to me. 那把椅子在我看来有点儿不结实。

It is a poor and politically unstable society. 这是一个贫穷落后且政局不稳的社会。

Synonyms: shaky (WEAK)unsound (WEAK)

An unstable person suffers from sudden and extreme changes in their mental and emotional state.

emotionally unstable 情绪上反复无常