"pertaining to the open sea, marine, oceanic" (as opposed to coastal), 1650s, from Latin pelagicus, from Greek pelagikos, from pelagos "sea, high sea, open sea, main." Beekes rejects the traditional derivation from PIE root *plak- (1) "to spread out, be flat" as without evidence and concludes instead that "the word rather seems to be Pre-Greek." In later use especially "living at or near the surface of the open ocean."