penguin (n.)

1570s, originally used of the great auk of Newfoundland (now extinct; the last two known birds were killed in 1844); the shift in meaning to the Antarctic swimming bird (which looks something like it, observed by Drake in Magellan's Straits in 1578) is from 1580s. The word itself is of unknown origin, though it often is asserted to be from Welsh pen "head" (see pen-) + gwyn "white" (see Gwendolyn), but Barnhart says the proposed formation is not proper Welsh. The great auk had a large white patch between its bill and eye. The French and Breton versions of the word ultimately are from English. A similarity to Latin pinguis "fat (adj.), juicy," figuratively "dull, gross, heavy," has been noted.