"fabulous water serpent," 1660s, from Latin Hydrus, from Greek hydros "water-snake" (see hydra). The constellation (attested by 1670s in English) was one of the 11 added to Ptolemy's list in the 1610s by Flemish cartographer Petrus Plancius (1552-1622) after Europeans began to explore the Southern Hemisphere.

1670s, from French hygiène, ultimately from Greek hygieine techne "the healthful art," from hygies "healthy, sound, hearty," literally "living well" (personified as the goddess Hygieia), from PIE *eyu-gwie-es- "having a vigorous life," from root *aiw-, *ayu- "vital force, life, long life, eternity; in the prime of life, young" (source of Latin aevus, English ever). The Greek adjective was used by Aristotle as a noun meaning "health." The difficult spelling in English is a relic of the struggle to render the Greek vowels into French.

"science of bodily humors," 1787, from French or German hygrologie, which are earlier, or from hygro- "wet, moist; moisture" + -logy.