What is the X in X-mas?
(cdr) /ku'dr/ or /kuh'dr/ The LISP operation that returns the right-hand node of a binary tree structure. In the typical case where the tree is used to store a list, the cdr is the tail of the list, i.e. all but the first element.
The instruction format of the IBM 7090 that hosted the original LISP implementation featured two 15-bit fields called the "address" and "decrement" parts. The term "cdr" was originally "Contents of Decrement part of Register". Similarly, "car" stood for "Contents of Address part of Register".