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"breast-shaped, resembling a breast or nipple," 1732, from Greek mastoeides "resembling a breast," from mastos "(woman's) breast" (see masto-) + -oeides "like," from eidos "form, shape" (see -oid). As a noun, 1800, from the adjective.
mastoid mas·toid (mās'toid')n. The mastoid process. adj.
Of or relating to the mastoid process, antrum, or sinuses.
Shaped like a breast or nipple.