Sombre and sombrero come from the same source: Latin umbra ‘shade’. Sombre entered English in the 18th century, from French, and is based on Latin sub ‘under’ and umbra. Sombrero, a hat that provides shade, is a Spanish word that comes from sombra ‘shade’, descended from umbra. The same root is shared by the English words umbrage (originally ‘shadowy outline’, then ‘grounds for suspicion’ and then ‘offence’), and umbrella (originally another name for a parasol or sunshade).