It also is a song by the Techno/alternative/punk/scene/scremo band BOTDF
which means- "Blood on the Dance Floor"
person 1- "Guess what? I started my period last night!!!"
person 2- "IDGAF"
person 1- "OMG i just heard the song IDGAF by BOTDF!!!"
person 2- "i love that song!!"
When somebody says this, they usually DO give a fuck. They're just trying to tell themselves that they don't, or to affirm their own confidence in themselves against whatever situation or person happens to be their opponent.
Also, a kind of insensitive way to responding to a situation when silence may be a more effective way to convey your meaning.
-I may or may not be a bully. IDGAF.
-I'm dying Squirtle.