and it sounds believable. Used in a UK music paper in 1988 or possibly not until 1990 (by David Quantick, comedy writer and critic, who has used it since, eg in Veep in 2012). After Trump ran for president, it started to be applied to him.
https://en.wiktionary.org/wiki/shitgibbon
https://web.archive.org/web/20170801205257/http://www.slate.com/blogs/browbeat/2017/02/13/the_origin_of_the_trump_insult_shitgibbon_revealed.html
It's an obvious insult, and the stress, syllables and vowels fit a common pattern:
Taylor Jones, a graduate student in linguistics from the University of Pennsylvania, wrote a post on his Language Jones blog that considers how shitgibbon fits an emerging pattern for obscene insults like douchewaffle, turdweasel, and jizztrumpet, all of which consist of a monosyllablic expletive plus a trochee (a stressed syllable followed by an unstressed syllable). Along with metrical concerns, Jones notes how vowels often repeat in such words, as in dickbiscuit, craprabbit, and spunkpuffin. Meanwhile, Jamie Reilly, director of the Memory, Concepts, Cognition Laboratory at Temple University, has been working with colleagues on a research project analyzing peoples judgments of such novel profanities. Clearly, shitgibbon and its kin will provide scholarly fodder for years to come.