The Collaborative International Dictionary of English v.0.48:

Forsake \For*sake"\, v. t. [imp. Forsook; p. p. Forsaken; p.
   pr. & vb. n. Forsaking.] [AS. forsacan to oppose, refuse;
   for- + sacan to contend, strive; akin to Goth. sakan. See
   For-, and Sake.]
   1. To quit or leave entirely; to desert; to abandon; to
      depart or withdraw from; to leave; as, false friends and
      flatterers forsake us in adversity.
      [1913 Webster]

            If his children forsake my law, and walk not in my
            judgments.                            --Ps. lxxxix.
                                                  30.
      [1913 Webster]

   2. To renounce; to reject; to refuse.
      [1913 Webster]

            If you forsake the offer of their love. --Shak.

   Syn: To abandon; quit; desert; fail; relinquish; give up;
        renounce; reject. See Abandon.
        [1913 Webster]

WordNet (r) 3.0 (2006):

forsake
    v 1: leave someone who needs or counts on you; leave in the
         lurch; "The mother deserted her children" [syn: abandon,
         forsake, desolate, desert]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "forsake":
   abandon, abdicate, back out, be unfaithful, beg off, break faith,
   cast off, chuck, cry off, deny, depart, depart from, desert,
   discard, drop out, evacuate, fail, flee, forgo, forswear, give up,
   go back on, have done with, jettison, jilt, leave, leave behind,
   leave flat, maroon, pass the buck, pull out, quit, quit cold,
   recant, reject, relinquish, renege, renounce, repudiate, resign,
   say goodbye to, shift the blame, shift the responsibility,
   stand down, surrender, take leave of, throw over, vacate, withdraw,
   yield