The Collaborative International Dictionary of English v.0.48:

Agist \A*gist"\, v. t. [OF. agister; [`a] (L. ad) + gister to
   assign a lodging, fr. giste lodging, abode, F. g[^i]te, LL.
   gistum, gista, fr. L. jacitum, p. p. of jac?re to lie: cf.
   LL. agistare, adgistare. See Gist.] (Law)
   To take to graze or pasture, at a certain sum; -- used
   originally of the feeding of cattle in the king's forests,
   and collecting the money for the same. --Blackstone.
   [1913 Webster]

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

AGIST, in contracts. The taking of other men's cattle on one's own ground at
a certain rate. 2 Inst. 643; 4 Inst. 293.