Earliest I know of stemmed from MSM trying to shut down Pizzagate.
Here's the earliest instance of its use I know of: https://www.nytimes.com/2016/11/21/technology/fact-check-this-pizzeria-is-not-a-child-trafficking-site.html
There's a bit of background here from the guy that created the original subreddit for pizzagate: https://steemit.com/pizzagate/@fuckcensorship/the-truth-has-no-bias-the-real-origins-of-r-pizzagate-and-the-pizzagate-pedogate-movement-part-1
Really good read for people that weren't around for Pizzagate as it was happening. This is the best summary of the situation I know of.
DJT took the term "Fake News" and flipped it on the MSM as an outcome of this Pizzagate fallout.