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 (cf. Middle English lights "the lungs," literally "the light (in weight) organs"). Also cf. pneumo-.
pulmonary pul·mo·nar·y (pul'mə-něr'ē, pŭl'-)
Of, relating to, or affecting the lungs.