archipelago: [16] Originally, archipelago was a quite specific term – it was the name of the Aegean Sea, the sea between Greece and Turkey. Derivationally, it is a compound formed in Greek from arkhi- ‘chief’ and pélagos ‘sea’ (source of English pelagic [17] and probably related to plain, placate, and please). The term ‘chief sea’ identified the Aegean, as contrasted with all the smaller lagoons, lakes, and inlets to which the word pélagos was also applied.
An ‘Englished’ form of the word, Arch-sea, was in use in the 17th century, and in sailors’ jargon it was often abbreviated to Arches: ‘An island called Augusto near Paros, in the Arches’, Sir T Roe, Negotiations 1626. A leading characteristic of the Aegean Sea is of course that it contains a large number of islands, and from the 16th century onwards we see a strong and steady move towards what is now the word’s main meaning, ‘large group of islands’.
The immediate source of the English word was Italian arcipelago, and some etymologists have speculated that rather than coming directly from Greek arkhipélagos, this may have been a sort of folk-etymological resuscitation of it based on a misunderstanding of Greek Aigaion pelagos ‘Aegean Sea’. => pelagic
archipelago (n.)
c. 1500, from Italian arcipelago "the Aegean Sea" (13c.), from Greek arkhipelagos, from arkhi- "chief" (see archon) + pelagos "sea" (see pelagic). The Aegean Sea being full of island chains, the meaning was extended in Italian to "any sea studded with islands." Klein, noting the absence of arkhipelagos in ancient or Medieval Greek (the modern word in Greek is borrowed from Italian) believe it is an Italian mistake for Aigaion pelagos "Aegean Sea" (Medieval Latin Egeopelagus), or influenced by that name.