The history of the English "postmeridian"

There was a (post-classical) Latin word postmeridianum from the 6ᵗʰ century that meant “after midday” which English first borrowed directly as postmeridian no later than 1583.

The OED says this of its history:

As noun < post-classical Latin postmeridianum the hours after midday, afternoon (6th cent.), use as noun (short for classical Latin postmerīdiānum tempus the time after midday) of neuter of classical Latin postmerīdiānus, adjective; as adjective < classical Latin postmerīdiānus (also in contracted form pōmerīdiānus) (adjective) of or occurring in the afternoon < post after (see post- prefix) + merīdiēs midday (see meridian adj.) + -ānus -an suffix (compare merīdiānus meridian adj.). With use as adjective compare antemeridian adj., and earlier pomeridian adj. With geological senses (see senses A. 2, B. 2) compare premeridian adj. In use as adverb perhaps erroneously for post meridiem adv.

The word postmeridian is now mainly used as an adjective:

Of or relating to the afternoon; occurring after noon or midday.

It also clocks in as a rare noun meaning

The hours after midday, the afternoon. Chiefly fig. Now rare.

Here’s one citation for that use:

1969 Hispanic Rev. 37 208 — The high noon of Charles the Fifth had declined into the somber post-meridian of Philip II.

The word is also sometimes used as an adverb meaning in or during the afternoon. For example:

1996 New Eng. Q. 69 397 — Turkey's ire builds up through the morning and, post-meridian, is vented.