The Collaborative International Dictionary of English v.0.48:

Probabilist \Prob"a*bil*ist\, n. [Cf. F. probabiliste.]
   [1913 Webster]
   1. One who maintains that certainty is impossible, and that
      probability alone is to govern our faith and actions.
      [1913 Webster]

   2. (Casuistry) One who maintains that a man may do that which
      has a probability of being right, or which is inculcated
      by teachers of authority, although other opinions may seem
      to him still more probable.
      [1913 Webster]