Merriam-Webster Collegiate Dictionary - oyster

 noun  Usage: often attributive  Etymology: Middle English oistre, from Anglo-French, from Latin ostrea, from Greek ostreon; akin to Greek ostrakon shell, osteon bone — more at osseous  Date: 13th century  1.  a. any of various marine bivalve mollusks (family Ostreidae) that have a rough irregular shell closed by a single adductor muscle and include commercially important shellfish  b. any of various mollusks resembling or related to the ~s  2. something that is or can be readily made to serve one's personal ends the world was her ~  3. a small mass of muscle contained in a concavity of the pelvic bone on each side of the back of a fowl  4. an extremely taciturn person  5. a grayish-white color