IDGAF is an internet chat slang term which can be expanded as "I don't give a fig". You say IDGAF to someone when you really don't care about what the other person is up to. The activities of the other person do not hold any value for you. When someone has no stake in your life, whatever he does will not impact you and you can simply say, "IDGAF".
IDGAF is a very common slang term which is used during internet chatting. IDGAF is uttered when one is furious and angry and does not feel like giving heed or weightage to what someone else has said and done. The expression IDGAF shows indifference and no care and concern. Don't use this term for someone who is really helpful and is in fact your well-wisher. Well-wishers and friends can criticize you at times, but they do that for your good. IDGAF can be abusive and hurt someone immensely who thinks about your interests too.