Word or phrase that everyone knows or has heard of, but no one knows its underlying meaning

What word or phrase can describe terms like:

• Wi-Fi
• AM/FM
• AM/PM
• RSVP
• "etc."
• "i.e."

and so on? In other words, what word or phrase could be used to describe a word or phrase that "everyone knows the word, but no one knows what it means/stands for/et cetera?"

Anachronisms

Which means ‘words belonging to a different period’.

The origin of which, (some!) people have either forgotten about or don’t know about.

I suggest this, as most of the words you mentioned come from an earlier time such as ‘id est’ and different languages such as French ‘repondez s’il vous plait’ or latin ‘et cetera’. Words of a ‘forgotten origin’ or ‘out of their time’.

As such they are ‘anachronistic’ which is from ‘ana’ - backwards and ‘khronos’ - time.

Little words which are having a teeny timewarp moment.

https://dictionary.cambridge.org/dictionary/english/anachronism