"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-, *ster- "to spread," related to stornynai "to spread out" (see structure, n.), on the notion of the chest as broad and flat, as opposed to the neck. Related: Sternal.