Lisp was originally implemented on the IBM 704 computer, in the late 1950s. The 704 hardware had special support for splitting a 36-bit machine word into four parts, an "address part" and "decrement part" of 15 bits each and a "prefix part" and "tag part" of three bits each.
Precursors to Lisp included the functions which took a machine address as an argument, loaded the corresponding word from memory, and extracted the appropriate bits.
car (short for "Contents of the Address part of Register number"), and
cdr ("Contents of the Decrement part of Register number").