The word 'sternum' entered English in the mid-17th century via Late Latin, where it was used for the breastbone. The Latin term was itself borrowed from the Greek 'sternon', which broadly meant 'chest, breast, or breastbone'. This Greek root likely descends from the Proto-Indo-European base ster-, meaning 'spread out, extend, be stiff', a concept that aptly describes the broad, flat structure of the bone. Thus, the modern anatomical term retains the ancient reference to the chest's central, expansive plate.

The word is a simplex loanword from Greek via Latin. It does not contain separable, meaningful morphemes in its modern English spelling.