The Collaborative International Dictionary of English v.0.48:

Thryfallow \Thry"fal`low\, v. t. [Perhaps fr. thrice + fallow.
   Cf. Trifallow.]
   To plow for the third time in summer; to trifallow. [R.]
   [Written also thrifallow.] --Tusser.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thrifallow \Thri"fal`low\ (thr[imac]"f[a^]l`l[-o]), v. t.
   See Thryfallow, and Trifallow. [R.] --Tusser.
   [1913 Webster]