The latest from Ben Zimmer — "A New Breakthrough in the History of the “S—gibbon: The Insult’s Originator Steps Forward", Slate 2/13/2017.
Following up on a comment by David Quantick on Ben's Strong Language Blog post, Ben found this passage in the 1/13/1990 issue of the New Musical Express, in which David Quantick and Steven Wells "imagine Morrissey of the Smiths (nickname: the Mozzer) and Mark E. Smith of the Fall in the year 2000":
’Tis the Mozzer and Mark E Smith! Yes, in the year 2000, Sir Morrissey del Manc and Shitgibbon Smith will be tired old buckers, fit for the scrapheap ever since some student NME reader got to see their poetry part of the English GCSE and finished so-called “serious rock” for, in the words of Alice Cooper, “EVAH!”
Apparently the term may have been used in other Quantick & Wells NME columns as early as 1988, though these have yet to be found.