The Collaborative International Dictionary of English v.0.48:

Obligable \Ob"li*ga*ble\, a.
   Acknowledging, or complying with, obligation; trustworthy.
   [R.]
   [1913 Webster]

         The main difference between people seems to be, that
         one man can come under obligations on which you can
         rely, -- is obligable; and another is not. --Emerson.
   [1913 Webster]