FFI stands for _Foreign Function Interface_.

FFI stands for Foreign Function Interface. Available in at least one Emacs fork (SXEmacs) and to GNU Emacs via a library by Christopher Wellens. Wellens explains why it did not form part of GNU Emacs earlier on:

[RMS] was concerned that people might use it to make proprietary extensions to the popular editor.

As of 2010, RMS wrote:

We gave GCC a plug-in interface last year, after developing a method to make it safer. So now it is ok to do this in Emacs as well.