"device which indicates atmospheric humidity," 1660s, from hygro- "wet, moist; moisture" + -scope. It indicates the presence of moisture but not the amount (which is measured by a hygrometer). Related: Hygroscopic.
word-forming element meaning "wet, moist; moisture," from Greek hygros "wet, moist, fluid; weak, soft, flexible." Beekes says possible cognates include Old Norse vokr (accusative vokvan) "moist, wet;" Latin uvidus, udus.