The Spanish word 'costilla' (meaning 'rib') comes from the Latin word 'costa', also meaning 'rib', combined with the Spanish diminutive suffix '-illa'. This formation literally means 'little rib', though in modern Spanish it's simply used as the standard word for 'rib' without any diminutive connotation.

A simpler related Spanish word is 'costa', which means 'coast' or 'shore'. While it might seem strange that 'coast' and 'rib' are related, both words come from the same Latin root 'costa'. The connection makes sense when you think of the coastline as the 'side' or 'edge' of the land, similar to how ribs form the side of the ribcage.

English speakers might recognize the connection to words like 'coastal', 'coast', and 'costal' (relating to the ribs). These all share the same Latin ancestor 'costa'. While English took these words primarily in the sense of 'shore/edge', the 'rib' meaning was preserved in medical terms like 'costal'. The word 'cutlet' also comes from this root, as it originally referred to a cut of meat from the side or ribs of an animal.