More mysterious epenthesis
Next question: why is someone from St Kitts in the Caribbean known as a Kittitian kɪˈtɪʃn?
Answer: I don’t know, and I suspect the OED doesn’t really know either, though it suggests that Kittitian is modelled on Haitian. (But Kitts : Kittitian is not really like Haiti : Haitian.)
And why are purveyors of tobacco known as tobacconists? Or for that matter people from Toronto as Torontonians? While members of LASSO, the Linguistic Association of the Southwest (US), according to Ryan Denzer-King, are addressed as Lassovians, and, as Nigel Greenwood points out, old boys of Stowe (public school) are simply known as Stoics.
Nigel Greenwood suggests that Shanghainese, from Shanghai, is presumably by analogy with Chinese (although I have to point out that the base form China contains an n, whereas Shanghai doesn’t, or at least not in the right place). In the east Asia area there are also Java – Javanese, Sunda – Sundanese, and Bali – Balinese, all with an n of no obvious origin unless indeed China – Chinese is somehow responsible.
The OED speculates that tobacconist, with -n- inserted between tobacco and -ist, is “perh. suggested by such words as Platonist, with etymological n”. For Torontonian it merely says “f. Toronto, capital of the province of Ontario in Canada + -n- + -IAN”.
*(the phrase that proves that two positives make a rather negative evaluation)