The Collaborative International Dictionary of English v.0.48:

Trifallow \Tri"fal`low\, v. t. [imp. & p. p. Trifallowed; p.
   pr. & vb. n. Trifallowing.] [Pref. tri- + fallow. Cf.
   Thryfallow.]
   To plow the third time before sowing, as land. --Mortimer.
   [1913 Webster]