sternum (n.)

"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.