1540s, "subjoined addition to a document or book," from Latin appendix "an addition, continuation, something attached," from appendere (see append). Used for "small outgrowth of an internal organ" from 1610s, especially in reference to the vermiform appendix. This sense perhaps from or influenced by French appendix, where the term was in use from 1540s.
appendix ap·pen·dix (ə-pěn'dĭks)
n. pl. ap·pen·dix·es or ap·pen·di·ces (-dĭ-sēz')
A supplementary or an accessory part of an organ or a structure of the body.
The vermiform appendix.
A small saclike organ located at the upper end of the large intestine. The appendix has no known function in present-day humans, but it may have played a role in the digestive system in humans of earlier times. The appendix is also called the vermiform appendix because of its wormlike (“vermiform”) shape.