c. 1500, from Italian arcipelago "the Aegean Sea" (13c.), from arci- "chief, principal," from Latin archi- (see arch-) + pelago "pool; gulf, abyss," from Medieval Latin pelagus "pool; gulf, abyss, sea," from Greek pelagos "sea, high sea, open sea, main" (see pelagic).
The elements of the word are Greek, but there is no record of arkhipelagos in ancient or Medieval Greek (the modern word in Greek is borrowed from Italian), so the word perhaps is an Italian compound or an alteration in Italian of Medieval Latin Egeopelagus, from Greek Aigaion pelagos "Aegean Sea." The Aegean being full of island chains, the meaning was extended in Italian to "any sea studded with islands" (a sense attested in English from c. 1600) and to the islands themselves. Related: Archipelagian; archipelagic.