A sanitized or movie-edited substitute for mother fucker
The film "Die Hard: With a Vengeance" was released in 1995
staring Samuel L. Jackson (as Zeus Carver) and Bruce Willis
(as John McClane); but this film was later edited for TV...
While the term "mellon farmer" probably appeared in other
edited films, Jackson's character made it quite funny and
memorable ... even though the original unedited movie was
far better ... the editors got a bit lucky, this time.
An example line from the edited film:
Zeus: Now that's low, even for a white mellon-farmer like you.