Plaintive is an adjective for describing someone or something with a pleading, sorrowful, desperate tone. If you have ever heard the plaintive howl of a wolf, then you know what we are getting at here.
A plaint, as in complaint, is an expression of sorrow or grief. This word has also been bent a little at the ends to become plaintiff or complainant — the sufferer — in a lawsuit. So, whether you are hearing a plaintive tone in a courtroom, at a funeral, or in the wild (as in an animal's plaintive howl), you can be assured that someone or something desires something desperately.