Can you back up what you said with any link or quotation? I did not find any mention of the word "axiom" in McCarthy's 1960 paper ("Recursive functions of symbolic expressions and their computation by machine"). Google for "axiomatic system lisp" pops up my own comment above as the first result.
I doubt this is in any way an established terminology, sounds again just like a very sloppy metaphor on part of the author - are complex pieces build from simple primitive pieces enough to call for "theorems" and "axioms" ?
