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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.