FERG is a word that is spelled and carefully pronounced as to not offend the gay community. It can be spelled either 'ferg' or 'furg', and is a branch-off from the word 'fag'.
The word fag
is defined as "a homosexual person; a stupid person.'
The word 'ferg/furg' simply means dumbass, retard, douchebag, and stupid fuck, in nicer terms.
It is usually screamed at another person in such a way that it sounds like the word fag
but there is really that underlying sound that makes it furg.