The Collaborative International Dictionary of English v.0.48:

Mainpernable \Main"per*na*ble\, a. [OF. main hand + pernable,
   for prenable, that may be taken, pregnable. See
   Mainpernor.] (Law)
   Capable of being admitted to give surety by mainpernors; able
   to be mainprised.
   [1913 Webster]

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

MAINPERNABLE. Capable of being bailed; one for whom bail may be taken;
bailable.