pulmonaryyoudaoicibaDictYouDict
pulmonary: [18] Latin pulmō meant ‘lung’ (it was related to Greek pleúmōn ‘lung’, ultimate source of English pneumonia). From it was formed the adjective pulmōnārius, which English adopted at the beginning of the 18th century as pulmonary.
pulmonary (adj.)youdaoicibaDictYouDict
1704, from French pulmonaire and directly from Latin pulmonarius "of the lungs," from pulmo (genitive pulmonis) "lung," cognate with Greek pleumon "lung," Old Church Slavonic plusta, Lithuanian plauciai "lungs," all from PIE *pleu- "to flow, to float, to swim" (see pluvial).

The notion perhaps 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 Middle English lights "the lungs," literally "the light (in weight) organs"). Also see pneumo-.