The Collaborative International Dictionary of English v.0.48:

Impound \Im*pound"\, v. t. [imp. & p. p. Impounded; p. pr. &
   vb. n. Impounding.]
   To shut up or place in an inclosure called a pound; hence, to
   hold in the custody of some authority such as police or a
   court; as, to impound stray cattle; to impound an illegally
   parked car; to impound a document for safe keeping.
   [1913 Webster +PJC]

         But taken and impounded as a stray,
         The king of Scots.                       --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impound
    v 1: take temporary possession of as a security, by legal
         authority; "The FBI seized the drugs"; "The customs agents
         impounded the illegal shipment"; "The police confiscated
         the stolen artwork" [syn: impound, attach, sequester,
         confiscate, seize]
    2: place or shut up in a pound; "pound the cows so they don't
       stray" [syn: impound, pound]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "impound":
   annex, attach, beleaguer, beset, besiege, blockade, bottle up,
   bound, box in, box up, cage, chamber, check, cloister, close in,
   collectivize, commandeer, communalize, communize, compass, confine,
   confiscate, constrain, contain, coop, coop in, coop up, cordon,
   cordon off, cork up, corral, crib, detain, distrain, encage,
   encircle, enclose, encompass, enshrine, expropriate, fence in,
   garnish, hedge in, hem in, hold, hold in custody,
   hold in restraint, house in, immure, impress, imprison,
   incarcerate, include, inhibit, jail, keep in, keep in custody,
   keep in detention, kennel, leaguer, levy, mew, mew up, nationalize,
   pen, pen in, pen up, pocket, pound, press, quarantine, rail in,
   replevin, replevy, restrain, restrict, seal up, sequester,
   sequestrate, shackle, shrine, shut in, shut up, socialize, stable,
   surround, wall in, wrap, yard, yard up