Back to the thread.

It isn't fye, it is fie. Like many old words in English, this has several meanings.

(1) True. (ie True, man, true)
(2) Predestined.
(3) A fairy. Don't quote me if you intend to sing this meaning in an UK pub.
(4) A facetious exclamation of disapproval (ie Crap, man, crap)

The Dict. of Archaic Words (1-2-3) and Collins (4)

I've always understood that I'm singing (4) but after a few pints (1) also works.