ˈfluːɪd ˌʌnˈsaʊnd ʌnˈsteɪbəɫ subject to change; variable a fluid situation fraught with uncertainty everything was unstable following the coup affording no ease or reassurance a precarious truce highly or violently reactive sensitive and highly unstable compounds disposed to psychological variability his rather unstable religious convictions suffering from severe mental illness of unsound mind lacking stability or fixity or firmness unstable political conditions the tower proved to be unstable in the high wind an unstable world economy