Very interesting Phil, you learn something new every day.
I never knew that those words meant anything, but they did if you were an acient celt:
Quote from article Phil linked to on wikipedia:
Charles Mackay, proposes in The Gaelic Etymology of the Languages of Western Europe that the seemingly meaningless string of syllables "Fa fe fi fo fum" is actually a coherent phrase of ancient Gaelic, and that the complete quatrain covertly expresses the Celts' cultural detestation of the invading Angles and Saxons:
Fa from faich (fa!) "behold!" or "see!"
Fe from Fiadh (fee-a) "food";
Fi from fiú "good to eat"
Fo from fogh (fó) "sufficient" and
Fum from feum "hunger".
Thus "Fa fe fi fo fum!" becomes "Behold food, good to eat, sufficient for my hunger!"