The Collaborative International Dictionary of English v.0.48:

Unavoidable \Un`a*void"a*ble\, a.
   1. Not avoidable; incapable of being shunned or prevented;
      inevitable; necessary; as, unavoidable troubles.
      [1913 Webster]

   2. (Law) Not voidable; incapable of being made null or void.
      --Blackstone.
      [1913 Webster]

   Unavoidable hemorrhage (Med.), hemorrhage produced by the
      afterbirth, or placenta, being situated over the mouth of
      the womb so as to require detachment before the child can
      be born.
      [1913 Webster] -- Un`a*void"a*ble*ness, n. --
      Un`a*void"a*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unavoidably
    adv 1: by necessity; "the situation slid inescapably toward
           disaster " [syn: inescapably, ineluctably,
           inevitably, unavoidably]