The Collaborative International Dictionary of English v.0.48:

entrench \en*trench"\, v. t.
   1. (Mil.) To surround with a trench or with intrenchments, as
      in fortification; to fortify with a ditch and parapet.
      Same as intrench.
      [1913 Webster]

   2. to establish in a position from which dislodgement is
      difficult; to place firmly in a strong position.
      [PJC]

   3. To cut in; to furrow; to make trenches in or upon.

   Syn: intrench. [1913 Webster]

WordNet (r) 3.0 (2006):

entrench
    v 1: fix firmly or securely [syn: entrench, intrench]
    2: impinge or infringe upon; "This impinges on my rights as an
       individual"; "This matter entrenches on other domains" [syn:
       impinge, encroach, entrench, trench]
    3: occupy a trench or secured area; "The troops dug in for the
       night" [syn: entrench, dig in]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "entrench":
   arm, armor, armor-plate, bank, barge in, barricade, battle, bed,
   blockade, break in, break in upon, bulwark, burst in, butt in,
   castellate, charge in, come between, confirm, crash, crash in,
   crash the gates, creep in, crenellate, crowd in, cut in, deep-dye,
   define, dig in, edge in, elbow in, embattle, embed, encroach,
   engraft, engrave, establish, etch, fence, fix, foist in, fortify,
   found, garrison, ground, horn in, impact, impinge, implant, impose,
   impose on, impose upon, impress, imprint, infiltrate, infix,
   infringe, ingrain, inscribe, insinuate, interfere, interlope,
   interpose, intervene, intrude, invade, irrupt, jam, lodge, man,
   man the garrison, mine, obtrude, pack, palisade, plant, press in,
   print, push in, put on, put upon, root, rush in, seat, set, set in,
   settle, slink in, slip in, smash in, sneak in, squeeze in, stamp,
   steal in, stereotype, storm in, strengthen, throng in, thrust in,
   trench, trespass, wall, wedge, work in, worm in