As explained by part time chemist and full time pimp Katt Williams, Fuck It is the chemical compound in marijuana that, when smoked, brings about higher brain activty in the user which allows said user to notice what is really important in life.
I thought it was a bad thing the eletric company shut down power in my home, but then I smoked a couple of joints, got some Fuck It in my system and didn't care. Ain't like I don't got some candels I've been waiting to burn!