The word 狼心狗肺 is formed by combining the characters for "wolf" (狼) and "dog" (狗) with "heart" (心) and "lungs" (肺) into a fixed four-character phrase, where the literal meaning of "wolf heart and dog lungs" serves as a metaphor to describe a person who is vicious, ungrateful, or morally corrupt, based on the negative traits traditionally associated with these animals.