1704, from L. pulmonarius "of the lungs," from pulmonem (nom. pulmo, gen. pulmonis) "lung," cognate with Gk. pleumon "lung," O.C.S. plusta, Lith. plauciai "lungs," all from PIE *pleu- "to flow, to float, to swim" (see pluvial). 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. (cf. M.E. lights "the lungs," lit. "the light (in weight) organs").