This is probably a transferred etymology: στερεός, στερρός "solid, hard" was the etymon commonly assumed for another name of the chest, στέρνον. The etymology of the latter was transferred to the synonym στῆθος, owing to the phonetic similarity of the words which share the initial sequence [st] (see στέρνον / στερεός). In the Gudianum, the preceding notice is Στέρνον, παρὰ τὸ στῶ, στείω· τὸ ἐντελὲς στέον, στερόν· καὶ πλεονασμῷ τοῦ ν, στέρνον, where στέρνον is derived from ἵστημι, which is the regular etymon of στῆθος: this could have played a role in the transfer.