Definition of water frog in US English:

water frog

noun

  • Any frog that lives in water; (in later use) especially any of a group of European green frogs that includes the marsh frog Rana ridibunda and the pool frog R. lessonae and hybrids between these, including the edible frog (formerly known as R. esculenta).

Origin

Old English; earliest use found in Chrodegang of Metz's Regula Canonicorum.