before vowels hydr-, word-forming element in compounds of Greek origin, meaning "water," from Greek hydro-, combining form of hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet"). Also sometimes a combining form of hydrogen.
word-forming element used in making names of devices for stabilizing or regulating (such as thermostat), from Greek statos "standing, stationary," from PIE *ste-to-, suffixed form of root *sta- "to stand, make or be firm." First used in heliostat "an instrument for causing the sun to appear stationary" (1742). Related: -static.
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<a href="https://www.etymonline.com/word/hydrostatic">Etymology of hydrostatic by etymonline</a>
Harper, D. (n.d.). Etymology of hydrostatic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/hydrostatic