Rat (n.) - From Pest To Pet.
A medium sized, long-tailed rodent.
Also used as an insult against traitors.
History & Etymology
Every resource I looked at about the origin of the word rat has been just a flood of cognates from Germanic languages. A cognate is a word with the same meaning that came from the same root but in a different language.
Here’s some of these cognates just to give you an idea of the variations between languages.
Ratto, Rotte, Rotta
And it’s generally just assumed that it entered English from Germanic, but looking at the Romance languages like Italian, French and Spanish all of the words sound similar.
Ratus, Raturus, Ratta, Ratte
One factor that makes the origin of the word so confusing is that there is no written evidence of the word existing before the end of the first millennium.
One theory is that the word entered the Germanic languages first as an imitation of the sound of gnawing that rats make with they chew on wood. Then it spread to from Germanic languages and into the Romance languages, but there is no solid evidence of where it originated so it could have been in reverse.
Even in PIE the word for “to scrape, scratch, or gnaw” was red-, and this word became the ultimate origin of the word rodent in English.