Why is the ninth month called September?
1540s, "belly fat," from Latin abdomen "belly," of unknown origin, perhaps from abdere "conceal," with a sense of "concealment of the viscera," or else "what is concealed" by proper dress. De Vaan, however, finds this derivation "unfounded." Purely anatomical sense is from 1610s. Zoological sense of "posterior division of the bodies of arthropods" first recorded 1788.
abdomen ab·do·men (āb'də-mən, āb-dō'mən)
The part of the body that lies between the chest and the pelvis and encloses the stomach, intestines, liver, spleen, and pancreas. Also called belly, venter.
abdominal adjective (āb-dŏm'ə-nəl)