E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Fi or Fie!
An exclamation indicating that what is reproved is dirty or indecent. The dung of many animals, as the boar, wolf, fox, marten, and badger, is called fiants, and the orificium anale is called a fi, a word still used in Lincolnshire. (Anglo-Norman, fay, to clean out; Saxon, afylan, to foul; our defile or file, to make foul; filth, etc.)
The old words, fie-corn (dross corn), fi-lands (unenclosed lands), fi-mashings (the dung of any wild beast), etc., are compounds of the same word.
I had another process against the dungfarmer, Master Fifi.Rabelais: Pantagruel, book ii. 17.