The Collaborative International Dictionary
Thryfallow
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.
Wiktionary
thryfallow
vb. (context obsolete English) To plough for the third time in summer.