Etimology of this word is from Spanish "pingüe" (fat; fatty). During the first circumnavigation (1519-1522) led by Magellan, the Spaniards discovered penguins in the Southern Atlantic. Mistaking them for some sort of fat geese, they described them as such (gansos pingües); hence "pinguino", which has passed to other languages.
Apparently, the word "penguin" originally referred to the great auk. Sources: Gaskell, Who Killed the Great Auk? (p.8); Salomon, Penguin-Pedia (p.12). Whether this means that "penguin" in English used to mean "great auk", I don't know. If it did, we should add an obsolete sense. Equinox ◑ 12:39, 30 April 2016 (UTC)