definitely not the first time he's used it. i heard him use it in a rally not too long ago, and i think its been in his tweets too

Origin of the term The origin of the term is traced to Charles Krauthammer, a conservative political columnist, commentator, and psychiatrist, who coined the phrase Bush derangement syndrome in 2003 during the presidency of George W. Bush. That "syndrome" was defined by Krauthammer as "the acute onset of paranoia in otherwise normal people in reaction to the policies, the presidency—nay—the very existence of George W. Bush".[10][11][12][13] The first use of the term Trump derangement syndrome may have been by Esther Goldberg in an August 2015 op-ed in The American Spectator; she applied the term to "Ruling Class Republicans" who are dismissive or contemptuous of Trump.[14] Krauthammer, in an op-ed commented that—in addition to general hysteria about Trump—the "Trump Derangement Syndrome" was the "inability to distinguish between legitimate policy differences and ... signs of psychic pathology."[13]

https://en.wikipedia.org/wiki/Trump_derangement_syndrome