etymology - The history of the English "postmeridian"

There's a question on English Language Learners that's been making the rounds recently, it's been on the Hot Network Questions list since January 5 this year and has attracted something like 36,000 views (all credit to tchrist's exhaustive and flawless answer for this extraordinary phenomenon).

I posted an answer, and from a comment, it seems I've done something terribly wrong, but I don't understand what exactly.

Until January 5, I believed that PM, P.M., p.m. or pm was short for post meridian and that it was spelled as two separate words. That's how I've always pronounced it. but it was pointed out to me that the correct Latin spelling is post meridiem. So, I edit the post and add the following reflection

… in English spelled postmeridian [emphasis mine].

Shortly after, a user posted the following comment

'Postmeridian' (single word, no space) may be English (although it is certainly antiquated by 'afternoon') but 'P.M.' is an abbreviation for the Latin phrase 'post meridiem'. There is no 'English spelling' of Latin phrases, there are only translations, but even then 'postmeridian' is actually worse of a translation than 'after noon', because of its archaic status. […]

Was I that wrong to say that ‘post meridiem’ is spelled ‘postmeridian’ in English? Is postmeridian an English translation? That doesn't make much sense to me, it looks very much like a loanword that's been anglicized (?) not translated.

The following definitions are from Merriam-Webster

post meridiem
adjective : being after noon —abbreviation PM
First Known Use: 1647

postmeridian
adjective : occurring after noon : of or relating to the afternoon the postmeridian hours of the day Origin and Etymology
Latin postmeridianus, from post- + meridianus meridian

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  Can someone explain if the original Latin expression consists two words and is spelled “post meridiem”, why is it spelled as a single word in English with a different suffix, dian, postmeridian?

  
  


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  Is postmeridian (or post meridian) a bastardization?

  
  


• What is the history of the word postmeridian and its spelling?

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Related (paullum) What is the proper name for "AM" and "PM"?

Answer

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.