The story
From both the action of taking or making post (“occupying an established military position”) and the practice of posting (“publicly announcing”) the names of such officers in the London Gazette.
Etymology from Wiktionary, CC BY-SA 4.0.
post-captain
Today it means: A captain of the 18th or 19th-century Royal Navy in command of a vessel, with his subsequent promotions almost always determined by seniority rather than merit or ability. Full definition →