As far as I know, 'xau' does not exist as a word in Mandarin
Chinese. Perhaps you meant 'xiao'. However, without knowing the
tone (or tune) of the word, it is impossible to accurately
translate it. 'xiao' has about 84 different meanings depending on
the tone and the context it is used in!
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actually, this person is asking what does "xau" mean in
portuguese, not in chinese.
anyway, "xau" is used frequently as an abbreviation for "tchau",
which in portuguese means "bye"