The Collaborative International Dictionary of English v.0.48:

Hark \Hark\ (h[aum]rk), v. i. [OE. herken. See Hearken.]
   To listen; to hearken. [Now rare, except in the imperative
   form used as an interjection, Hark! listen.] --Hudibras.
   [1913 Webster]

   Hark away! Hark back! Hark forward! (Sporting), cries
      used to incite and guide hounds in hunting.

   To hark back, to go back for a fresh start, as when one has
      wandered from his direct course, or made a digression.
      [1913 Webster]

            He must have overshot the mark, and must hark back.
                                                  --Haggard.
      [1913 Webster]

            He harked back to the subject.        --W. E.
                                                  Norris.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hark
    v 1: listen; used mostly in the imperative [syn: hark,
         harken, hearken]