Plaintive means expressing sorrow, mournful.
Plaintive comes under adjective.
Synonyms: sad, melancholy, lamenting, moaning.
Antonyms: happy, merry, joyful.
The author's early poems were plaintive, but after he fell in love they became joyful and lighthearted.
I hope our neighbors get home soon, the plaintive wails from their lonely dogs are keeping me up all night.