'mastoid' originates from Modern Latin, specifically the word 'mastoideus', which itself comes from Greek elements where 'mástos' meant 'breast' and '-oeidēs' meant 'like or in the form of'.

'mastoid' changed from Greek 'mástos' + suffix '-oeidēs' into Latin 'mastoideus' and later entered English in anatomical/medical usage as 'mastoid'.

Initially it meant 'breast-shaped' (literally 'like a breast'), but over time it evolved into its anatomical sense of 'a breast-shaped bony projection' (the mastoid process).