The story
From fetus + Latin papȳrāceus (“made of paper”), or could be considered an entirely Latin compound, consistent with medical naming.
Etymology from Wiktionary, CC BY-SA 4.0.
fetus papyraceus
Today it means: A vanishing twin that, rather than being completely reabsorbed, is compressed by its growing twin to a flattened, parchment-like condition. Full definition →