It originally had been used to describe a very real phenomenon, which was that there were shady, flyby night "news sites" that were created specifically to be spread virally over social media with very little respect to the actual truth. So we got stories from the "FreedomUSANews" that your old classmate from high school was plastering over his Facebook page talking about how "FBI is preparing to indict Hillary Clinton, who also only has three months to live!". Or to that basic effect. And you could do a basic fact check and within minutes debunk whatever nonsense was being promoted as "news." But most people were too lazy to do this. And if you did point out the fact checking, the person posting the "news" would simply attack the fact checker rather than consider the documentable facts. (Seriously--they *hate* Snopes. Hate them with a passion.)
This phenomenon heavily benefited Trump during the campaign, and was likely being bankrolled and supported by--ahem--certain foreign interests.
After the election, when people started asking why there was a level of distrust and reluctance for Hillary Clinton that persuaded just enough people not to vote for her to the point it influenced the electoral results. And people started talking about this "fake news" and its influence in getting Donald Trump elected.
Trump saw this, and because he didn't want people questioning his legitimacy, he choose to co-opt the term. So every time any legitimate media source--who unlike these "fake news" sites employs real identifiable people and does real journalism--published something not to Trump's liking, he'd should "fake news". And thus CNN, the Washington Post and the New York Times were placed on the same level as "RealPatriotsUniteNews". And eventually, Trump used the term "fake news" so often that it simply became meaningless and nothing more a joke, and people started to forget that "fake news" actually once referred to a very real thing.
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