It has no real English meaning. In the story, the giant chants this phrase so as to incite terror in listeners.

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!"[6]

It's a variation on a familiar passage in an old fairy tale, "Jack and the Beanstalk." It doesn't mean anything. In the fairy tale, Jack is hiding, and the giant yells, "Fee! Fi! Fo! Fum! I smell the blood of an Englishman! Be he alive or be he dead, I'll grind his bones to make my bread!" This little chant is familiar to most native English speakers.