The Collaborative International Dictionary of English v.0.48:

Twifallow \Twi"fal`low\, v. t. [imp. & p. p. Twifallowed; p.
   pr. & vb. n. Twifallowing.] [AS. twi- (see Twice) two +
   fallow.]
   To plow, or fallow, a second time (land that has been once
   fallowed).
   [1913 Webster]