The Collaborative International Dictionary of English v.0.48:

Incessable \In*ces"sa*ble\, a. [L. incessabilis; pref. in- not +
   cessare to cease.]
   Unceasing; continual. [Obs.] --Shelton. -- In*ces"sa*bly,
   adv. [Obs.]
   [1913 Webster]