Perdida literally means "lost (female) one". However, in the song's title phrase "¿Qué onda perdida?", it's a very common and informal Mexican slang way of saying, "What's up, stranger?" or "Long time no see!"
It's used to address someone you haven't seen or heard from in a while, as if they've been 'lost'. In this context, the singer uses it as a bold and familiar way to reconnect with his ex, mixing a bit of playful accusation with underlying affection.
Have you ever had an ex who just refuses to take a hint? That is the whole spirit of "Qué Onda Perdida" by Grupo Firme and Gerardo Coronel ("Jerry"). The title itself uses a cheeky piece of Mexican slang: "¿Qué onda perdida?" ["What's up, stranger?" or literally, "lost girl"]. It is the classic phrase people text someone they have not talked to in ages. Right from the start, the singer announces himself as "tu ex, el último de tus amores" [your ex, the last of your loves] and sends flowers with a direct order: smile, don't cry. His logic for showing up uninvited is hilariously bold. He noticed she forgot to block his number: "No me bloqueaste del WhatsApp y eso para mi es nuevo" [You didn't block me on WhatsApp and that's new for me]. To him, a missing block is basically an open invitation to rekindle the romance.
From there, the singer takes things up a notch from a simple text to full-blown doorstep drama. He tells her, "Te me alistas porque hoy te llevo la banda" [Get ready because today I'm bringing the brass band to you], swearing he will camp outside her house until she steps out. He insists on a face-to-face showdown to hear the painful truth directly: "pa' que me digas en la cara / Que ya no me quieres / Que tú no me extrañas / Y que alguien mejor, está ocupando tu cama" [so you tell me to my face / that you don't love me anymore / that you don't miss me / and that someone better is occupying your bed]. Even then, he laughs it off, saying nobody could possibly replace him. He calls her his "peleonera" [feisty fighter] and "batallosa" [hard to handle], showing that despite the cold shoulder, he is completely hooked on the chase.