This is one of those Trumpisms that derive from his profound ignorance, because he beat up some nerd to do literally every homework assignment in his life. A famous one is "Bigly," because he didn't understand when he first heard it that they were saying "Big League."

For a long time, I wondered what "very stable genius" meant, and it finally dawned on me. He must have heard someone described as a "veritable genius," and he decided to apply it to himself, and it garbled out as "very stable genius."