Green’s Dictionary of Slang

fi’penny n.

[? its original price]

(Aus.) a clasp-knife.

[Aus]Vaux Vocab. of the Flash Lang.
[UK]D. Sladen in Barrère & Leland Sl., Jargon and Cant I 363/1: Fipenny (thieves), a clasp knife. The term is in common use in Australia, where it was introduced by the convicts.
[UK]Farmer & Henley Sl. and Its Analogues.