The term "fake news" predates Trump's candidacy for president. It started when Obama signed a bill that ended an FDR policy that prevented the news media from knowingly spreading false information.
Immediately after the news media was legally allowed to lie, the term "fake news" appeared. And it came from the very same news media that is now lying their butts off and have been ever since Obama.
Trump just picked up on it pretty much like everyone else did. It is real easy to point out fake news but when the Democrats and their propaganda machine (the mainstream news media) want to push crap on us they lead with by calling any opposition, "fake news".