"breastbone," 1660s, from Greek sternon "chest, breast, breastbone" (in Homer, only of males), also "the breast as the seat of affections," related to stornynai "to spread out," from PIE *ster-no- "to stretch, extend," from root *stere- "to spread," on the notion of the chest as broad and flat, as opposed to the neck. Related: Sternal.
"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.