The Collaborative International Dictionary of English v.0.48:

Prescriptible \Pre*scrip"ti*ble\ (pr[-e]*skr[i^]p"t[i^]*b'l), a.
   [Cf. F. prescriptible.]
   Depending on, or derived from, prescription; proper to be
   prescribed. --Grafton.
   [1913 Webster]

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

PRESCRIPTIBLE. That which is subject to prescription.