Definition of arroyo in English arroyoPronunciation: /əˈrɔɪəʊ/ noun (plural arroyos) a steep-sided gully formed by the action of fast-flowing water in an arid or semi-arid region, found chiefly in the south-western US. Origin: mid 19th century: from Spanisharroyo in other Oxford dictionaries Definition of arroyo in the US English dictionary