posterus

Posterus, postera, posterum, built on post. The nominative singular masculine is rare to the point of being a textbook fiction; what you meet is the oblique cases, and above all postero die, "on the next day", which is how a chronicle moves from one entry to the next. In posterum means "for the future", and the plural posteri, used as a noun, means descendants or posterity.

The degrees are irregular and useful. Posterior gives English posterior; postremus is "last of all"; and postumus, "last-born", is the child arriving after the father's death. Late writers wrongly linked that last one to humus, "ground", and added an h, which is why English says posthumous and means something no Roman meant.