As a noun sternpost
is a timber or steel bar extending from the keel to the main deck at the stern of a vessel.
As an adjective sternmost is
closest to the stern, furthest sternward.
sternpost
English
Noun
(
en noun)
(nautical) A timber or steel bar extending from the keel to the main deck at the stern of a vessel.
References
* FM 55-501 Marine Crewman’s Handbook
sternmost
English
Adjective
(-)
Closest to the stern, furthest sternward.