Merriam-Webster's Collegiate® Dictionary, 11th Edition
SHEATH
noun (plural sheaths) see: shed Date: before 12th century a case for a blade (as of a knife), an investing cover or case of a plant or animal body or body part: as, the tubular fold of skin into which the penis of many mammals is retracted, b. the lower part of a leaf (as of a grass) when surrounding the stem, an ensheathing spathe, any of various covering or supporting structures that are applied like or resemble in appearance or function the sheath of a blade: as, sheathing 2, a woman's close-fitting dress that is usually worn without a belt, condom 1