Usually sounds like "Forgedaboudit" and spoken slightly loud to put an end to the subject.
Guido: Forget about it!
1) Sometimes means just what it says ie it would be beast for all concnered not to rememeber.
2) That someone or something has no chance of succeeding
2) The yankees to be the red sox - forget about it! They got no hope.
(A) You have just been forgiven a major trespass by the Don.
(B) The Don wants you to think you are forgiven, but really, you are about to die.
"Oh, faggot about it, thilly!" = The gay pimp (Peter Seller) version of "forget about it".