The Collaborative International Dictionary of English v.0.48:

Astern \A*stern"\, adv. [Pref. a- + stern.] (Naut.)
   1. In or at the hinder part of a ship; toward the hinder
      part, or stern; backward; as, to go astern.
      [1913 Webster]

   2. Behind a ship; in the rear. "A gale of wind right astern."
      --De Foe. "Left this strait astern." --Drake.
      [1913 Webster]

   To bake astern, to go stern foremost.

   To be astern of the reckoning, to be behind the position
      given by the reckoning.

   To drop astern, to fall or be left behind.

   To go astern, to go backward, as from the action of
      currents or winds.
      [1913 Webster]

WordNet (r) 3.0 (2006):

astern
    adv 1: stern foremost or backward; "the steamer went astern at
           half speed"
    2: at or near or toward the stern of a ship or tail of an
       airplane; "stow the luggage aft"; "ships with square sails
       sail fairly efficiently with the wind abaft"; "the captain
       looked astern to see what the fuss was about" [syn: aft,
       abaft, astern] [ant: fore, forward]
    3: (of a ship or an airplane) behind; "we dropped her astern on
       the end of a seven-inch manilla, and she laid comfortably on
       the ebb tide"

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "astern":
   a rebours, a reculons, abaft, aft, after, against the grain,
   anticlockwise, arear, ass-backwards, away, back, backward,
   backwards, baft, counterclockwise, fore and aft, fro, hindward,
   hindwards, in reverse, rear, rearward, rearwards, retrad,
   widdershins