A more publicly usable alternative for the word
Fucking.
Generally used as an Adjective, like to emphasize something.
The word Fuck is refferred to as the F-Word generally,
so "
Fucking" would be F-Word with an "ing",
so that would be F-ing, and this is pronounced as "Ef-ing" and this evolved into "Effing" or "Efing" being an alternative for
Fucking.
The "g" in "Effing" isn't required to get the desired pronounciation, so it is omitted, giving our result of "Effin" or "F-in"