Merced literally means "mercy," but in the phrase a su merced it conveys being "at someone’s mercy" or completely under their control.
Quevedo sings, "me usa a su merced" (she uses me at her mercy), playing with the homophones USA and usa to show how the girl both studies in the USA and "uses" him however she pleases. It’s a poetic, somewhat archaic word that rarely appears in modern lyrics, making it stand out and perfect for your Spanish vocabulary list.
Spanish hitmaker Quevedo sets the scene right in the opening lines: a girl returns home to her island after a grueling academic year studying at Columbia University in the United States. She is totally burned out by exams and expectations, and her friends convince her that this break is strictly for drinking, partying, and answering to no one. But her plan crashes the moment she locks eyes with him in a crowded club. Even though she acts tough and pushes him away playfully, her feelings slip out before she can stop them: "Se le escapó un 'te quiero' a la que no quería nada" [An 'I love you' slipped out from the girl who wanted nothing].
What makes the track so catchy is the ticking clock hanging over their heads. Both know that once August wraps up, she has to fly back to America, leaving them with an expiration date. Instead of pretending it will last forever, they cram every moment with memories—speeding down island roads at 180 km/h with reggaetón blasting, waking up early after staying out late, and trading kisses against bedroom walls. Quevedo drops plenty of casual island slang too, cutting words short like "ca' vez" for cada vez [each time] and making wordplay out of her degree: "Sé que estudia en la USA y me usa a su merced" [I know she studies in the USA and uses me at her mercy]. It is a whirlwind romance that proves summer flings hurt the most when you both swore you were too scared of commitment to fall in love.