2020's Trash Skill "Gacha" deals with an ability whose name is only understandable to the (Japanese?) audience and is possibly not otherwise explained, but the in-universe uninformative name being a plot point is the point I'm circling around.
From TVTropes: Orphaned Etymology:
2010's(?) The Stormlight Archive has the mysterious origin of the "Hound" part of "Axehounds", which is relevant to the wider plot.
The 2000 Interactive Fiction Shrapnel has a character saying:
Einstein
And the everything about being a user of that word reveals to the audience that things aren't what they seem.
So, what's the first time a character / world uses a word they shouldn't have access to given their worldbuilding, and it's plot relevant, and the in-universe reason for that word, if any, isn't immediately obvious to the audience?
In fact, I'm not sure The Stormlight Archive is an example. I don't know if the reader is supposed to know the reason before it's explained.