"presence of air in the pleural cavity," 1821, from French pneumothorax (1803), coined by French physician Jean Marc Gaspard Itard (1774-1838) from Greek pneumon "lung" (see pneumo-) + thorax.
pneumo-
pneumonia
pneumonic
pneumono-
pneumonoultramicroscopicsilicovolcanoconiosis
pneumothorax
pnyx
Po
*po(i)-
poach
poached