Thick green- or yellow-colored mucus that is produced as a result of a respiratory disease or infection and coughed up out of the mouth; phlegm. My grandmother always kept a pack of tissues in her pocket that she could spit her lung butter into after she had a coughing fit. Look, I get it—you're sick. But at least have the decency to rinse the lung butter down the drain after you spit it into the sink!