The Collaborative International Dictionary of English v.0.48:

Agister \A*gist"er\, Agistor \A*gist"or\, n. [Anglo-Norman
   agistour.] (Law)
   (a) Formerly, an officer of the king's forest, who had the
       care of cattle agisted, and collected the money for the
       same; -- hence called gisttaker, which in England is
       corrupted into guest-taker.
   (b) Now, one who agists or takes in cattle to pasture at a
       certain rate; a pasturer. --Mozley & W.
       [1913 Webster]