Acc'g. to the page where I looked up the quod erat demonstrandum, quorum means "of whom". How therefore did it get to be a noun?

quorum means "of whom"

And, it's plural.

How therefore did it get to be a noun?

(Semantic) shift happens!

And ignoramus was a verb. Also plural. First person, no less.

Ya swipes a word from some weirdo foreignian language and all the little footnotes and stuff on the back with the circles and errors and stuff kinda gets misread or ignored.

Ya swipes a word from some weirdo foreignian language

Yes, my favorites are bus, which is an abbreviation off the end of omnibus which in Latin means 'for every (body)', and, then you take the the auto off of automobile and jam it on the front for autobus; auto is from Greek where it means 'he; self'. Now that's one crazy mixed up word, yet nobody blinks as it rumbles by.

barbarismus?!
Graecimus??

placet is another example of such a conversion.

placet

Yes, as are Latin-to-English audio 'I hear', exit 's/he leaves', and video 'I see'.

Well--I don't know what placet* is, but thanks for the other examples. :-)
*plasset? placket? pluh-chet?

*plasset? placket? pluh-chet?
Plassay? Plasset gets my vote of assent.

i vote for placket

It is used in a phrase discovered carved on a rock near the site of an old Roman taberna in Britain:

ORES TABIT FORTIS ARARE PLACET ORESTAT

ORES TABIT FORTIS ARARE PLACET ORESTAT

A similar one I learned in Bonn; the inscription on an ancient Roman vase: DATIS IN PINCVLA POTVS COLONIA.

[Added forgotten extra word.]

It occurs to me that our modern use of the term "quorum" could be the remnant of longer phrase.

the term "quorum" could be the remnant of longer phrase

It is. It's from the legal Latin wording enumerating members of a commission: quorum vos ... unum esse volumus 'of whom we wish you to be one' (link).

Eau de Cologne? would it be asked too much to hope get from you (the formal or plural you) classici a translation for us, non latinists? These BIG LATIN WORDS I sure would like to know their meaning. Inscriptions beside pubs are intriguing, not to mention
the vase...

Dunno bout Nuncle z's but mine puzzled the Latin scholars. A little kid came along and read it off easy as cake.

thanks. after reading this I was easily able to read your 'latin' inscription.

That makes more sense than a single word derivation.

These BIG LATIN WORDS

As for Faldo's, I'll leave it to the gentle reader to figure it out, but mine, though it appears to be Latin, is actually in Kölsch, the dialect of the city of Cologne and environs: Dat iss en Pinkele Pott uss Cölle (That's a piss-pot from Cologne.).

Vale, I'd better let it rest a bit. BTW. ORES TABIT FORTIS Fortis is one of the Belgian-Dutch banks that was given a rest after crashing like so many of them. e.i. nationalized.