Definition of water cow in US English:

water cow

noun

  • 1A legendary or mythical animal associated with water; compare water bull.

  • 2Any of various large amphibious, aquatic, or water-loving mammals, as the water buffalo, hippopotamus, manatee, (female) waterbuck, etc.

Origin

Late 18th century; earliest use found in John Sinclair (1754–1835), agricultural improver, politician, and codifier of ‘useful knowledge’.