https://www.lamborghini.com/it-en/news/marcello-gandini-racc...
it was just a (most probably very local) common interjection/expletive by a mechanic (from Piedmont) that was chosen half way between a joke and because it sounded not-so-bad in English as well.
The literal meaning is "peste o contagio" (pox or contagion or disease) but it is actually used essentially in what would be "perbacco" o "accidenti" (which I believe would be in English "by jove!" or "damn!").
? Tuscan was the written standard for Italian since Petrarch and the Piedmontese conquest of Southern Italy was done by 1871. Admittedly colonization took a long time but it’s a very long time since anything but Tuscan has been used in official or prestige circumstances in Italian.
> official or prestige circumstances in Italian
Lamborghini differentiated itself from Ferrari in large part by being brash and in your face. Given what countach means I think it's pretty safe to say that they were not trying to come up with a prestigious name.
In some regions (both in the north and in the south) many people are - still today - functionally bilingual, i.e. they still use largely dialect (and that is their "native" language) and switch automatically to Italian when dealing with (say) authority/government officers/foreigners or when they travel outside their region.
But I believe that at the moment the majority of people from these regions are in the "reverse" mode, i.e. they normally speak Italian but switch to the dialect in case of need or with older relatives/known friends.