Certainly in Czech the two are related: tučňák = noun, penguin tučný = adjective, fat I don't know if that helps determine the origin of the English word, bu

Certainly in Czech the two are related: tučňák = noun, penguin tučný = adjective, fat I don't know if that helps determine the origin of the English word, but it's definitely funny to think that there’s a nation which calls penguins “fatties” :D