The Collaborative International Dictionary of English v.0.48:

Portionist \Por"tion*ist\, n.
   1. A scholar at Merton College, Oxford, who has a certain
      academical allowance or portion; -- corrupted into
      postmaster. --Shipley.
      [1913 Webster]

   2. (Eccl.) One of the incumbents of a benefice which has two
      or more rectors or vicars.
      [1913 Webster]