peninsula of southern Greece, from Latin, from Greek Peloponnēsos. The second element apparently is nēsos "island" (see Chersonese); the first element is said to be from Pelops, name of the son of Tantalus, who killed him and served him to the gods as food (they later restored him to life). The proper name is probably from pelios "gray, dark" (from PIE root *pel- (1) "pale") + ōps "face, eye" (from PIE root *okw- "to see"). But the association of the proper name with the peninsula name likely is folk etymology.
Related: Peloponnesian (late 15c. as a noun, "a native or inhabitant of the Peloponnesus"). The Peloponnesian War (1570s) was the great struggle for hegemony between Athens and her maritime empire and Sparta and her allies on the Peloponnesus, waged from 431 B.C.E. to 404 B.C.E.