The Origins and Meaning of the Grau Surname:
German and Jewish (Ashkenazic)
A nickname for someone with gray hair or a gray beard, from the German grau ‘gray.’
A topographic name for someone who lived near a canal giving access to the sea, Occitan grau (Latin gradus ‘step’).
From Old French grau denoting a type of agricultural fork with curved tines (apparently of Germanic origin), hence a metonymic occupational name for a maker, seller, or user of such implements.
A topographic name from grau (Latin gradum).
From a reduced form of the common medieval personal name Guerau (see the Gerald Surname).