[kazinator]

The term FEXPR most likely refers to the fact that in the original LISP, a FEXPR is an interpreted function: it contains expressions which the interpreter must process. All the built-in functions in LISP which do the same thing are called FSUBR, and are written in machine language. For instance LET has a binding to a FSUBR, whereas a user-defined LET-like operator would be a FEXPR. While that adds a new construct to the language, it must itself be interpreted to evoke its semantics.