infant-from Latin infans; in (neg.) + fans speaking (fans is the participle). In Roman law infant means a child not old enough to talk...
Cretin-Old French-chretien, a Christian. Perhaps used first as a perjorative term by the ancient Romans because to them the first Christians were stupid enough to "turn the other cheek" when attacked by mobs or by the lions in the Roman amphitheater.
Meconium-from Greek mekonion meaning the poppy juice obtained from pressing the whole plant which gives a thick Juice of black, greenish-brown color. The intestinal content of the newborn infant has a similar consistency and appearance and so Galenadopted the term for the content of the bowels of newborn infants. (Meconism means the opium habit).
lcterus-from Greek ikteros, a yellow bird, probably the golden thrush or a species of oriole. Pliny relates that if a person suffering from jaundice looks at a yellow bird, the bird will die and the patient recovers. Icterus was first used in pediatrics by Ludwig von Buhl (1816-1880) when he described icterus neonatorum.