means A soft-bodied marine mollusc that lives inside a hard, hinged shell, prized both as food and as the occasional producer of pearls.

from From Old French 'oistre,' which came from Latin 'ostrea,' itself borrowed from Greek 'ostreon' — all rooted in a sense of 'shell' or 'bone-hard covering,' a cousin of Greek 'osteon' (bone) and 'ostrakon' (a potsherd or shell used as a voting token in ancient Athens). So the name simply means 'the hard-shelled thing,' which is exactly what you notice before you ever notice the pearl.