Originated in the locker room after zero hour by Audrey, it is used in conjunction with "day" as in "FYIDC day" It means to not care what you're wearing, Audrey uses it for days where she threw things in her band bag and wears her Birkenstocks.
Man, I got my Birkenstocks. That can only mean one thing. IT'S FYIDC DAY!
a phrase often used when referring to the dapper young individual known as Darren Criss. also a populartumblr account devoted to the latest news on said individual.