(non-native speaker)
Is there a reason why the English language has “special” words for a specific topic, like related to court (plaintiff, defendant, warrant, litigation), elections/voting (snap election, casting a ballot)?
And in other cases seems lazy, like firefighter, firetruck, homelessness (my favorite), mother-in-law, newspaper.
Many of those words aren’t actually limited to those use cases, but they are used there because they have very specific meanings. A plaintiff, for example, is the person lodging a complaint. Doesn’t have to be a legal complaint, but in legal terms it makes it very clear who we’re talking about.
Others are just expressions. A snap election is just when you call an election earlier than the scheduled one. It’s essentially a nickname for something more complicated. Same goes with casting a ballot. It just means to toss your vote in for the count. It’s just the expression that stuck because it sounds fancier than just saying “voting.”
I think the OP was wondering why that person wouldn’t just be called a complaintlodger. Like with firefighter.
A quick “rule” is to see how old the word/concept is. “plaintiff” would have existed almost as long as the English legal system came into being, or probably even older to the court of Assizes pre-12th century.
Whereas firefighter as a profession might have only become a word after the establishment of fire departments by insurance companies, which I think might have been a 19th century development.
Or complainer…