lungyoudaoicibaDictYouDict[lung 词源字典]
lung: [OE] Lungs, insubstantial air-filled sacs, got their name because they weigh so little. It comes ultimately from Indo-European *lnggh-, a variant of which produced English light ‘not heavy’. In prehistoric Germanic this became *lungg-, which over the centuries has differentiated to German lunge, Dutch long, Swedish lunga, and English lung. The similarly motivated use of the word lights for ‘lungs’ dates from the 12th century; it is now restricted to ‘animals’ lungs used as food’, but it was formerly a general term.
=> light[lung etymology, lung origin, 英语词源]
lung (n.)youdaoicibaDictYouDict
"human respiratory organ," c. 1300, from Old English lungen (plural), from Proto-Germanic *lungw- (cognates: 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" (cognates: 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. Compare 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.