The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]