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amphibian (adj.)

1630s, "having two modes of existence; of doubtful nature," from Greek amphibia, neuter plural of amphibios "living a double life," from amphi "of both kinds" (see amphi-) + bios "life," from PIE root *gwei- "to live."

Formerly used by zoologists to describe any sort of animal at home on land and in the water, including crocodiles, walruses, beavers, seals, hippopotami; the restriction to the class of animals between fishes and reptiles with life cycles that begin in water and mature on land is from 1835; Amphibia has been used a zoological classification in this sense since c. 1819.

amphibian (n.)

"one of the class of animals between fishes and reptiles, having gills and living in water in the early stage of life, later living on land," 1835; from amphibian (adj.). Amphibia was used in this sense from c. 1600.