The Collaborative International Dictionary of English v.0.48:

Issuable \Is"su*a*ble\, a. [From Issue.]
   [1913 Webster]
   1. Leading to, producing, or relating to, an issue; capable
      of being made an issue at law. --Burrill.
      [1913 Webster]

   2. Lawful or suitable to be issued; as, a writ issuable on
      these grounds.
      [1913 Webster]

   Issuable plea (Law), a plea to the merits, on which the
      adverse party may take issue and proceed to trial.
      [1913 Webster]

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

ISSUABLE, practice. Leading or tending to an issue. An issuable plea is one
upon which the plaintiff can take issue and proceed to trial.