means The bony enclosure formed by the ribs, sternum, and spine that surrounds and protects the heart, lungs, and other organs of the chest.

from A plain compound of two old English words bolted together: "rib," from Old English "ribb," cousin to German "Rippe" and tracing back to a Proto-Germanic root, and "cage," which took the long road through Old French "cage" from Latin "cavea," a hollow place or enclosure (the same root that gives us "cave"). So the word literally pictures the ribs as a hollow cage — a relatively modern pairing, anatomical English deciding the metaphor was too good to pass up.