Fundo comes from the verb fundir, which literally means 'to melt' or 'to cast metal'. However, in modern slang, it takes on a whole new meaning.
In the song, Peso Pluma sings, "Te fundo el cel", which translates to 'I'll blow up your cell phone' with calls. This is a very current and common expression in Spanish to describe calling someone repeatedly, making it a perfect example of how language evolves with technology.
Mexican corrido tumbado star Peso Pluma teams up with Tito Double P on "Daño" to talk about a sudden, messy breakup. It took just one night for things to fall apart: "Sólo una noche bastó pa' que me dejara" [Just one night was enough for her to leave me]. The ex is totally fed up, telling him "que a la chingada" [to go to hell], an extremely common Mexican swear phrase to say you are completely done. He promises not to blame her, but immediately admits he cannot keep his word: "Tal vez hoy tomo y ya bien loco / Te fundo el cel, la llamada" [Maybe today I'll drink and, once I'm crazy drunk, I'll blow up your phone with calls]. The Mexican slang phrase fundir el cel literally means to melt or burn out the phone, meaning he plans to spam her with late-night calls until the device practically overheats.
The chorus hits with the weird emptiness of coming home to a quiet house: "¿Por qué se siente tan extraño / Llegar y no sentir tus labios?" [Why does it feel so strange / To arrive and not feel your lips?]. He is soaking handkerchiefs in tears ("Llené de lágrimas el paño" [I filled the cloth with tears]) and stopped smiling. Even though he begs her to take his hand and escape this strange world together, he drops the ultimate toxic confession right at the end: "Te amo aunque me hagas tanto fucking daño" [I love you even though you hurt me so fucking much]. It is chaotic, drunk-dialing energy wrapped up in a heartbreak track.