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"
"That's very amazing !!"
More Emphasised but not publicly acceptable:
"That's fucking amazing !!"
More Emphasised AND publicly acceptable:
"That's effin amazing !!"
child: Who is Bush?
me: Some moron dipshit cunt faggot asshole sonofabitch pussy dickhead cocksucker bastard that says retarded homo bullshit all the time!