Pliny the Elder, a Roman writer, documented the earliest account pertaining to the inhabitants of the peninsula around the mid-first century AD, referring to them as the Catharrei, a designation which may have derived from the name of a prominent local settlement. A century later, Ptolemy produced the first known map to depict the peninsula, referring to it as Catara. The map also referenced a town named “Cadara” to the east of the peninsula. The term ‘Catara’ (inhabitants, Cataraei) was exclusively used until the 18th century, after which ‘Katara’ emerged as the most commonly recognised spelling. Eventually, after several variations – ‘Katr’, ‘Kattar’ and ‘Guttur’ – the modern derivative Qatar was adopted as the country’s name.
In Standard Arabic, the name is pronounced [ˈqɑtˤɑr], while in the local dialect it is [ˈɡitˤar].
Source: Qatar – Wikipedia