tertian（adj.）
late 14c., tercian, "occurring every other day" (in reference to fevers), from Latin tertianus, from tertius "third, a third," from the root of tres "three" (see three).
A counting by the ancient system of inclusive reckoning (now we would say every second day); compare nones, bissextile, quinzane, etc.. Also the name of an old liquid measure of a third of a tun. Ornithology uses tertial "of the third rank or row" (1836) in reference to bird feathers.
该词起源时间：14世纪末