Meaning

noun process of the temporal bone behind the ear at the base of the skull

adjective of or relating to or in the region of the mastoid process

adjective relating to or resembling a nipple

originBorrowed from New Latin mastoīdēs, from Ancient Greek μᾰστοειδής (măstoeidḗs, “like a breast”), from μᾰστός (măstós, “a woman's breast”) + -ειδής (-eidḗs, “-like, -oid”).

Etymology from Wiktionary, CC BY-SA 3.0