From another website:
Thank you for your question. The term 'jaffa' which is used to describe an unplayable delivery is fairly commonplace now. But strangely enough no one seems to know where it came from. I know that the BBC has also asked people to try and come up with evidence of the origin of the term.
The only thing I can think of is that 'jaffa' in Hebrew means 'beauty' - which would tie in nicely with an unplayable ball being described as a beauty. The only flaw with my theory is that some cricket commentator/ writer would have to be well versed in Hebrew, which seems pretty unlikely.