Descuido literally translates to an 'oversight', 'neglect', or 'moment of carelessness'. It's a word that can be used in many situations, but KAROL G gives it a very playful and strategic meaning in this song.
She sings, "Estoy esperando el primer descuido" (I'm waiting for the first slip-up), cheekily suggesting she's waiting for her crush to make a 'mistake' or have a lapse with his current girlfriend. This clever use of descuido reveals her confident personality and makes it her perfect opportunity to swoop in.
Ever locked eyes with someone across a party and immediately thought: Why didn't I meet you first? Colombian superstar KAROL G lays out that exact frustration here. She watches her crush out on the floor and asks the ultimate question: "¿Qué hubiera sido? Si antes te hubiera conocido" [What would have happened? If I had met you earlier]. To her, the answer is obvious. You would be slow-dancing with her instead of just being buddies ("no como amigos" [not as friends]). She knows he is currently taken, but she is completely unfazed by his girlfriend. In fact, she bluntly points out that while the other woman is pretty, "Ella es tímida y yo no" [She is shy and I am not] and "Hoy estás jangueando con ella / Pero después tal vez no" [Today you are hanging out with her / But later maybe not]. Notice the Puerto Rican street slang jangueando, which comes straight from the English verb "to hang out."
KAROL G plays the part of the hopeful schemer with hilarious confidence. She jokes that she needs a dating manual to win him over ("que me pasen el manual" [pass me the manual]), only to immediately jump to wedding plans: "Mi nombre suena bien con tu apellido" [My name sounds good with your last name]. She says she is just waiting for the girlfriend to slip up ("el primer descuido") so she can introduce him as her husband. It is bold, slightly unhinged, and totally fun to sing along to when you are secretly crushing on someone off-limits.