A nonsense rhyme, usually heard as part of the Jack The Giant Killer fable.

The words are nonsense and the phrase has no allusory meaning. Apart from when quoting Shakespeare or Jack the Giant Killer, there’s little reason ever to use it. As with many early English expressions the spelling is arbitrary and there are many variants in print:

Fee, fa, foh, fum
Fee, fi, fum
Fie, fih, foh, fum
Fee, fa, fum and so on.