Opacaba comes from the verb opacar, which means to overshadow, dim, or make something less bright or prominent. It's a less common verb in everyday conversation, making it stand out.
In the song, the lyrics describe how a
"FKU" follows a woman shaking off a bad relationship and turning the breakup into her comeback. She used to get dressed up for a guy who slowly dimmed her light — "Pero le tocó un loco / Que poco a poco / To' se lo opacaba" ("But she got stuck with a crazy one / who little by little / dimmed it all") — and when he won't let her go, she settles it with one line that covers both sides: "Lo insultó y le dijo fuck you... También le dijo thank you" ("She cursed him out and said fuck you... she also told him thank you") — fuck you for the damage, thank you for the lesson. From there she's back out until sunrise, waterproof makeup on in case the night gets sweaty — "Hoy se maquilló de más / Y se lo puso waterproof / Por si baila de más" ("Today she put on extra makeup / and made it waterproof / in case she dances too much") — done being anyone's plan B. Dímelo Flow, Divino and iZaak trade the verses over a perreo beat, signing off with their own names as the closing tag.