The Collaborative International Dictionary of English v.0.48:

Withernam \With"er*nam\, n. [AS. wi[eth]ern[=a]m; wi[eth]er
   against + n[=a]m a seizure, fr. niman to take.] (Law)
   A second or reciprocal distress of other goods in lieu of
   goods which were taken by a first distress and have been
   eloigned; a taking by way of reprisal; -- chiefly used in the
   expression capias in withernam, which is the name of a writ
   used in connection with the action of replevin (sometimes
   called a writ of reprisal), which issues to a defendant in
   replevin when he has obtained judgment for a return of the
   chattels replevied, and fails to obtain them on the writ of
   return. --Blackstone.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

WITHERNAM, practice. The name of a writ which issues on the return of
elongata to an alias or pluries writ of replevin, by which the sheriff is
commanded to take the defendant's own goods which may be found in his
bailiwick, and keep them safely, not to deliver them to the plaintiff until
such time as the defendant chooses to submit himself, and allow the
distress, and the whole of it, to be reprieved, and he is thereby further
commanded that he do return to the court in what manner he shall have
executed the writ. Hamm. N. P. 453; 2 Inst. 140; F. N. B. 68, 69; 19 Vin.
Ab. 7; 7 Com. Dig. 674; Grotius, 3, 2, 4, n. 1.