Definitions of astern:

• adverb:   (of a ship or an airplane) behind

  Example: "We dropped her astern on the end of a seven-inch manilla, and she laid comfortably on the ebb tide"

• adverb:   stern foremost or backward

  Example: "The steamer went astern at half speed"

• adverb:   at or near or toward the stern of a ship or tail of an airplane

  Example: "The captain looked astern to see what the fuss was about"