IDGAF
short for 'I Don't Give A Fuck'

there are two forms of it- a response to any insult thrown at you, or a mental state of mind normally accompanied by a reckless state of acting like yourself unashamedly.
her: you suck
him: if i do, it not gonna be you. IDGAF

P. 1: Have you seen Derrick today? Hes acting like we're at a party, not like we're at school. talkin crazy shit and everything
P. 2: Hes clearly in an IDGAF state of mind today.