E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
By far the most probable derivation of this word is that given by Bochart, from the Phnician Baratanic (country of tin), contracted into Bratan. The Greek Cassiterides (tin islands) is a translation of Baratanic, once applied to the whole known group, but now restricted to the Scilly Isles. Aristotle, who lived some 350 years before the Christian era, calls the island Britannic, which is so close to Bratanic that the suggestion of Bochart can scarcely admit of a doubt. (De Mundo, sec. 3.)
Pliny says, Opposite to Celtiberia are a number of islands which the Greeks called Cassiterids (evidently he means the British group). Strabo says the Cassiterids are situated about the same latitude as Britain.
Great Britain consists of Britannia prima (England), Britannia secunda (Wales), and North Britain (Scotland), united under one sway.