hippopotamus
Concise Oxford English Dictionary © 2008 Oxford University Press:
hippopotamus /ˌhɪpəˈpɒtəməs/
▶noun (pl. hippopotamuses or hippopotami /-mʌɪ/) a large thick-skinned semiaquatic African mammal, with massive jaws. [Hippopotamus amphibius and Choeropsis liberiensis (pygmy hippopotamus).]
– origin ME: via L. from Gk hippopotamos, earlier hippos ho potamios, ‘river horse’.
'hippopotamus' also found in these Oxford entries:

