Wikipedia:

The word ain't can be used in both speech and writing to catch attention and to give emphasis, as in "You ain't seen nothing yet," or "If it ain't broke, don't fix it." Merriam-Webster's Collegiate Dictionary gives an example from film critic Richard Schickel: "the wackiness of movies, once so deliciously amusing, ain't funny anymore."

(Which is exactly how it's used in "ceteris ain't paribus". See also this post by Geoff Nunberg.)