Here's how the word evolved:
1. "Planctus" (Latin) - meaning "complaint" or "lamentation."
2. "Planctus" (Medieval Latin) - used to refer to a formal complaint or accusation.
3. "Plaintiff" (Middle English) - adopted from the Medieval Latin usage, signifying the person making a formal complaint in a legal context.
So, essentially, a plaintiff is someone who "laments" or "complains" about a wrong done to them, leading to a legal case.