sternocleidomastoid (adj.)

"collarbone," 1610s, from French clavicule "collarbone" (16c.), also "small key," from Medieval Latin clavicula "collarbone" (used c. 980 in a translation of Avicenna), special use of classical Latin clavicula, literally "small key, bolt," diminutive of clavis "key" (from PIE root *klau- "hook"); in the anatomical sense a loan-translation of Greek kleis "key, collarbone," which is from the same PIE source. So called supposedly from its function as the "fastener" of the shoulder. Related: Clavicular.

"breast-shaped, teat-like, resembling a (female) 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.