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"containing no water," 1819, a modern coinage from Greek an-, privative prefix (see an- (1)), + hydor "water" (see water (n.1)). Greek did have anhydros "waterless," used of arid lands or corpses that had not been given proper funeral rites.
anhydrous an·hy·drous (ān-hī'drəs)adj. Without water, especially water of crystallization.