If you do something plaintively, you’re doing it in a way that expresses sadness or melancholy. When you get tired of playing fetch, dogs will look up at you plaintively. Just try to resist those big sad eyes, just try!
The root of the word plaintively is the French word plainte, meaning “lamentation.” It’s also the origin of the word plaintiff, meaning a person who brings a lawsuit against someone else. The person has a complaint, something to lament or be sad about. If she argues her case plaintively enough, however, the judge might be convinced by her sorrow and rule in her favor.