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