The names CAR and CDR weren't invented for Lisp or FLPL. They came from assembly language mnemonics on the IBM 704, where the first Lisp interpreter was implemented.

The original Lisp CAR and CDR operations used the machine-level instructions with those names on the IBM 704.

Cons cells were stored in single words of the 36-bit memory and 36/38-bit registers, and the CAR and CDR operations accessed the "address" and "decrement" 15-bit parts of a word, respectively.