lung

Old English lungen — plural, because humans have two — comes from Proto-Germanic *lungwô, which derives from the same root as the adjective 'light' (not heavy). The connection was made by ancient butchers and cooks who observed that animal lungs, unlike liver or kidney, float in water. The lung was the light organ, the floating thing.

This observation was universal across Indo-European languages. Greek pneumon (lung) relates to pneuma (air, breath). Latin pulmo (lung) is connected to a root meaning 'to swim' or 'to float.' The Greek and Latin names emphasize the function — breathing, air — while the Germanic name emphasizes the physical property — lightness, buoyancy. Two cultures, two observations, the same organ.

Lungs were not always understood to be about oxygen — that concept was not available until Joseph Priestley isolated oxygen in 1774 and Antoine Lavoisier demonstrated in 1783 that breathing was oxidation. Before that, physicians understood that lungs processed air into something vital, but the mechanism was obscure. Galen believed lungs cooled the heart and absorbed 'pneuma' — vital spirit.

The phrase 'lights' — still used by butchers for animal lungs sold as food — preserves the Old English sense of lightness. When a Victorian butcher sold 'lights,' he was selling the floating, light organs. The word lung and the word 'light' (the adjective) share their Proto-Germanic ancestry. The organ's name remembers the day it floated in a butcher's basin.