costilla
rib
costa
coast
+
-illa
diminutive suffix
Costilla means rib. Think of it as a small costa0 (coast0) with the diminutive -illa1, as ribs form a coast-like curve in the body.
noun
- ribA rib, one of the curved bones of the rib cageSe rompió una costilla en el accidente. // He broke a rib in the accident.