IPA: /ˈpleɪn.tɪv.li/
KK: /ˈplentɪvli/
In a way that expresses sadness or sorrow.
She spoke plaintively about her lost pet.
Plaintively → It is formed from "plaint" (from Old French *plaint*, meaning a lament or complaint) and "-ively" (a suffix forming adverbs, meaning in a manner of). The word "plaintively" means in a manner that expresses sorrow or lamentation.
Think of someone expressing a 'lament' ('plaint') in a sorrowful way — that's why plaintively means to do so in a sad manner.