The Brief: AF is an abbreviation for "as f**k." AF is usually placed after a word for emphasis.

DEEP DIVE
AF is an abbreviation for “as f**k.” It is usually placed after a word for emphasis.
Wholesome af pic.twitter.com/9ecboWxONi
— DUA LIPA (@DUALIPA) July 17, 2018
I swear some people annoying af and irritating af
— De'arra Taylor (@dearra) July 16, 2018