(nautical) Behind; toward the stern relative to some other object or position; aft of.
The captain stood abaft the wheelhouse.
1773, James Cook, An Account of a Voyage Around the World, Book 3, Chapter 5, in John Hawkesworth (ed.), An Account of the Voyages Undertaken by the Order of His Present Majesty: for Making Discoveries in the Southern Hemisphere, London: W. Strahan and T. Cadell, Volume 3, p. 558,
[…] we could hear the water rush in a little abaft the foremast, about three feet from the keel: this determined me to clear the hold intirely.