The word placenta, first used in a 1638 anatomy textbook, was borrowed from the New Latin phrase placenta uterina, meaning "uterine cake", because the circular, flat shape of the organ was thought to resemble a traditional Roman flat cake. The placenta part of that traces to Ancient Greek plakoenta, the accusative of plakoeis, "flat". That traces to the earlier word plax, which could be translated as "cake", "tablet", "plain", or pretty much anything flat and broad, and that, through a Proto-Hellenic word sounding like pluks, traces to the Proto-Indo-European reconstruction pleh, also meaning "flat" (and the etymon of flag, plank, fluke, and more). In biology, the word placenta refers to the part of flowering plants where ovules form - that simply comes from the anatomical definition.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.