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.
This song drops you right into the middle of a sudden breakup. It only took one night for his partner to decide she was done. She says she's "cansada / Muy agotada" (tired / very exhausted) and, to put it bluntly, tells him to get lost. The singer tries to act cool about it, promising "No habrá reproches" (There will be no reproaches), but he immediately takes it back with a funny, honest line: "Bueno, no prometo nada" (Well, I don't promise anything). He knows exactly what he's going to do: get drunk and blow up her phone.
The whole mood shifts to pure heartbreak in the chorus. He asks, "¿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?). It's that empty feeling of a broken routine. He admits he doesn't smile anymore and begs her to run away with him from this "mundo extraño" (strange world). The song's title, which means "Damage," comes from the final, killer line: "Te amo aunque me hagas tanto fucking daño" (I love you even though you do me so much fucking damage). He knows the relationship hurts him, but he can't let go.