mastoid (adj.)

before vowels mast-, word-forming element meaning "female breast, mammary gland," from Greek mastos "woman's breast," from madan "to be wet, to flow," from PIE *mad- "wet, moist, dripping" (source also of Latin madere "be moist;" Albanian mend "suckle;" see mast (n.2)).

word-forming element meaning "like, like that of, thing like a ______," from Latinized form of Greek -oeidēs (three syllables), from eidos "form," related to idein "to see," eidenai "to know;" literally "to see" (from PIE *weid-es-, from root *weid- "to see"). The -o- is connective or a stem vowel from the previous element. Often implying an incomplete or imperfect resemblance to the thing indicated.

medical Latin, from sterno- "sternum," Greek sternon "breast, breastbone," or Latin sternum (see sternum) + Latinized Greek kleis (see clavicle) + mastoid.