Source : Webster's Revised Unabridged Dictionary (1913)

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.