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]