It's more than usual looking to me, since like many here I'm often looking into what words where used for, I'm also looking at what they were used of, and hence encounter the use quite often.
Of the great many cases in etymonline.com alone, a great many are exactly as used here.
Oxford have no qualms about using it:
The word was used of Irish peasants dispossessed by English settlers and living as robbers, and extended to other marauders especially in the Scottish Highlands. (Definition of Tory).
And nor do I.
To specifically address:
Does "to be used OF" mean "to be used FOR":
The phrasing specifically reflects the relationship between a word and what it represents. If you agree with the comments above that it seems like a forced attempt to sound erudite, then you could use for or perhaps about or "to refer to".
I for one don't, so while it's not a phrasing I often find myself using (even here, when what words were used of is a particularly common topic), I won't shy from it, either.