Tremendi is a colloquial, shortened form of "tremendo," which means "tremendous" or "enormous." It's a slang term often used in Latin American Spanish to describe something exceptionally good, impressive, or large.
In this song, LIT killah uses it to compliment a woman's physique, specifically her "booty." He sings, "Ese booty, mami, 'tá tremendi," which translates to "That booty, mami, is tremendous." This word choice adds a playful, informal, and highly engaging tone to the lyrics, making it memorable and unique to the song's style.
LIT killah and FMK's "Ese Mensaje" is all about that intense, almost obsessive longing for someone who's a bit mysterious and maybe even a little dangerous. Our singer is totally captivated, saying things like "Mami, sé que vienes del infierno" [Mommy, I know you come from hell] and "La muerte me persigue, dime cuándo vamo' a verno'" [Death chases me, tell me when we're going to see each other]. It's like he knows she's trouble, but he can't get enough!
He's arrived in the city, eagerly waiting for "ese mensaje" [that message] from her, but it's not coming through: "No debo tener WiFi, porque no hay señal de ti" [I must not have WiFi, because there's no signal from you]. He's completely smitten, a "fan de tu cuerpo y tu tatuaje" [fan of your body and your tattoo], and he's convinced she thinks of him even when she's with someone else. He's ready to spoil her, offering to take her to "Chanel o Fendi," and even compares them to "Priscilla, yo tu Presley." It's a fun, high-energy track about being totally hooked on someone and just desperate for them to come back.