'avellaneous' originates from Late Latin 'avellanēus' (or Latin 'avellānus'), ultimately from 'avellana' meaning 'hazelnut' (a name connected with the ancient town 'Avella', noted for its hazelnuts).

'avellanēus' (Late Latin) passed into Medieval/early Modern English in forms such as 'avellaneous' or 'avellaneus' and eventually stabilized in modern English as 'avellaneous'.

Initially it literally meant 'of or relating to the hazelnut', but over time the sense narrowed to describe the color—'hazel-colored' or 'nut-brown'—rather than the nut itself.