A plaintive sound is high and sad, like a small cry or a thin song that wants comfort. We use it for voices, cries, music, and sometimes a look — a plaintive violin, a plaintive “are you still there?” on the phone. It is not a legal word, even though it shares old roots with plaintiff. The feeling is more “aching and a little helpless” than “angry” or “formally complaining.” It belongs to stories and careful description more than to street talk; you would not call a loud protest plaintive. Though actually people do stretch it a little: a plaintive email can mean the writer sounds hurt and pleading, not that the sound is high.