What is the X in X-mas?
"human respiratory organ," c.1300, from Old English lungen (plural), from Proto-Germanic *lungw- (cf. Old Norse lunge, Old Frisian lungen, Middle Dutch longhe, Dutch long, Old High German lungun, German lunge "lung"), literally "the light organ," from PIE *legwh- "not heavy, having little weight; easy, agile, nimble" (cf. Russian lëgkij, Polish lekki "light;" Russian lëgkoje "lung," Greek elaphros "light" in weight; see also lever).
The notion probably is from the fact that, when thrown into a pot of water, lungs of a slaughtered animal float, while the heart, liver, etc., do not. Cf. also Portuguese leve "lung," from Latin levis "light;" Irish scaman "lungs," from scaman "light;" Welsh ysgyfaint "lungs," from ysgafn "light." See also lights, pulmonary. Lung cancer attested from 1882.
Either of the two saclike organs of respiration that occupy the pulmonary cavity of the thorax and in which aeration of the blood takes place. It is common for the right lung, which is divided into three lobes, to be slightly larger than the left, which has two lobes.