Placenta
late 17th century: from Latin, from Greek plakous, plakount- ‘flat cake’, based on plax, plak- ‘flat plate’.
wiktionary
Borrowed from New Latin placenta uterina(“uterine cake”), from Latin placenta(“flat cake”), because of the flat round shape of the afterbirth.
etymonline
placenta (n.)
1670s of plants, "part of the ovary of flowering plants which bears the ovules," 1690s of mammals, "organ of attachment of a vertebrate embryo or fetus to the wall of the uterus or womb of the female," from Modern Latin placenta uterina "uterine cake" (so called 16c. by Italian anatomist Realdo Colombo), from Latin placenta "a cake, flat cake," from Greek plakoenta, accusative of plakoeis "flat," from plax "flat, flat land, surface, plate," from PIE root *plak- (1) "to be flat." So called from the shape.
