isles off Cornwall, of unknown origin. Pliny has Silumnus, Silimnis. Perhaps connected with the Roman god Sulis (compare Aquae sulis "Bath"). The -y might be Old Norse ey "island" The -c- added 16c.-17c. "[A]bout the only certain thing that can be said is that the c of the modern spelling is not original but was added for distinction from ModE silly as this word developed in meaning from 'happy, blissful' to 'foolish.'" [Victor Watts, "Cambridge Dictionary of English Place-Names," 2004].
