Correct spelling for the English word "nasau" is [nˈasa͡ʊ], [nˈasaʊ], [n_ˈa_s_aʊ] (IPA phonetic alphabet).
The word nasau does not have a clear etymology as it appears to be a misspelling or a variant spelling of the word nausea. Nausea originates from the Latin word nausea meaning seasickness or feeling sick. It was borrowed into English during the 16th century. However, nasau itself does not have a documented history or origin as a distinct word. It is possible that it is a dialectal variation or an accidental misspelling of nausea.