Its not hard to think of things that meet both of those features that are still better mnemonics for things that are common-language descriptions of what the function yields (e.g., cfc/csc or c1c/c2c for "cons first cell, cons second cell")

car and cdr are mnemonics related to the particular instruction set of the first computer on which Lisp happened to be implemented, which is great for people with a background with system programming on that system, but not so good for anyone else.

They're entrenched enough in the language that they are worth changing unless you are intentionally building a new Lisp-like language from scratch with some other main motivation and the change is just a side issue -- and it is clear that having something that doesn't imply list semantics for cons cells like first/rest does is good -- but car/cdr aren't a particularly good pair.

I know car/cdr history, and didn't like it. Only after trying clojure[1], I had a weird sensation that I was missing car/cdr and that writing in scheme felt better.

If I tried to explain deeper, Lisp focus on recursion over cons-lists was so foreign to the mainstream paradigms, that car/cdr, as most used operations, became symbolic axioms in my mind.

[1] which goes all in on the intuitive, human, pragmatic side of things, throwing away some old lisp idioms.

fst/rst, although having the difference in the middle also seems aesthetically appealing to me.

Neat, but unfortunately the equivalent of CADR (second item in list: first-of-rest) is then FRST, which I think would just be too easy to misinterpret as meaning "first" when reading or writing code in a hurry.

I don't know if cr shortcuts are still used often nowadays (SICP advocates for abstraction layers, accessor functions, CL has destruct-bind, and ml uses pattern matching to dig into data).

Nor do I -- but I'm pretty sure that those shortcuts are pretty much the only good reason for caring much about having names that resemble CAR/CDR in the way FST/RST do. (For any other purpose, I think FIRST/REST are obviously better than FST/RST.)

Good catch.

While writing a toy lisp I looked for other names with structural qualities, actually not names, more symbols since these weren't words.

Things like Oo/oO, xo/ox or _/__.