Rata de dos patas literally translates to "two-legged rat". It's a classic and powerful insult in Spanish, used to describe a despicable, worthless person by comparing them to the lowest of vermin.
This phrase is famously the title of an iconic song by Mexican singer Paquita la del Barrio. In "MAMIII", Karol G directly references Paquita, using this ultimate insult to put her ex in his place, making it a fiery and culturally rich expression of female empowerment.
MAMIII is a sharp, funny kiss-off track where Becky G and KAROL G shut the door on a toxic ex who will not stop calling. Becky G starts off by saying she sent his fake love on vacation without a visa ("Sin visa ni pasaporte / Mandé tu falso amor de vacaciones") and even tossed her phone in the trash because of his bad energy ("No me vuelvas a llamar / Que hasta boté el celular"). She is moving on without looking back, joking that she left him a review with zero stars ("Te dejé el review, no te puse ni una estrella") and telling him that if he wants her new number, he can dial "1-800-jódete" (1-800-screw-you).
KAROL G jumps in with even heavier hits, quoting the famous Mexican singer Paquita la del Barrio with the classic insult "Rata de dos patas" (two-legged rat). She uses blunt Colombian slang like gonorrea to call out how badly he treated her, mocking the fact that while he is out with someone new, he is still crying and thinking about her ("Andas comiéndote a otra, pero estás pensando en mí"). It is an energising breakup track packed with bold humor, reminding you that when something is toxic, you throw it out and never look back.