We’re used to thinking phonetically, especially with the type of grassroots language forms used in current times on social media. Visitors to the UK often complain how our spelling rarely seems to match the normal pronunciation. We Brits are used to it, of course, having had to integrate so much ‘alien’ vocabulary from invaders …
Logically we might expect Venecian to be the right spelling to describe people/products originating from Venice.
Venetian, however, is basically the adjective from VENETO, the wider region, rather than Venice, the small city isolated in its lagoon. Due practically to the location of marble quarries where ingredients for the plaster are found – those fragments formerly discarded as useless for major building work.
The specific language of this region is also called Venetian (or Venetan): used by almost 5 million inhabitants, and plus those who emigrated to other continents. It has a unique grammar, so not merely a dialect of the mother tongue Italian.
This word entered English usage during the 15th century with military and upper-class travel and new trade routes. Once established, very hard to change…
*Compare station: of Latin/Roman origin (statio), passing through France where it became station, (and in Spanish estacion). But the French call Venice Venise, and Venetian, Venitien. We understand that vocabulary items are passed through a cultural filter and settle into the linguistic sediment.
Nobody will be penalised for failing to spell it the conventional way – but you may be interested to know.