1650s, "air-spirit," from Modern Latin sylphes (plural), coined 16c. by Paracelsus, originally referring to any race of spirits inhabiting the air, described as being mortal but lacking a soul. Paracelsus' word seems to be an arbitrary coinage, but perhaps it holds a suggestion of Latin silva and Greek nymph, or Greek silphe "a kind of beetle," but French etymologists propose a Gaulish origin. The Century Dictionary comments that, "to occultists and quacks like Paracelsus words spelled with -y- look more Greek and convincing." The meaning "graceful girl" first recorded 1838, on the notion of "slender figure and light, airy movement" [OED].

"of the woods," 1570s, from Middle French sylvain (1530s), from Latin silvanus "pertaining to wood or forest" (originally only in silvanae "goddesses of the woods"), from silva "wood, woodland, forest, orchard, grove," of unknown origin. The unetymological -y- is a misspelling in Latin from influence of Greek hylē "forest," from which the Latin word formerly was supposed to derive.