CAR means Content of Address part of Register and CDR means Content of Decrement part of Register. They were tied to the 36-bits nature of the first LISP machin

CAR means Content of Address part of Register and CDR means Content of Decrement part of Register. They were tied to the 36-bits nature of the first LISP machines, with 15 bits to CAR and 15 bits to CDR (plus 2 bits for tags IIRC). So its not just naming keys in a structure, those had a very low level meaning related the the implementation of the CONS cell structure in hardware.