The Collaborative International Dictionary of English v.0.48:

Illeviable \Il*lev"i*a*ble\, a.
   Not leviable; incapable of being imposed, or collected. [R.]
   --Sir M. Hale.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ILLEVIABLE. A debt or duty that cannot or ought not to be levied. Nihil set
upon a debt is a mark for illeviable.