Webster’s Revised Unabridged Dictionary · 1913 Instable In·sta′ble, a. [L. instabilis: cf. F. instable. See In- not, and Stable, a., and cf. Unstable.] Not stable; not standing fast or firm; unstable; prone to change or recede from a purpose; mutable; inconstant. ‹ Instability Instableness ›