Mosca muerta literally translates to "dead fly," but in Spanish, it's a common idiom used to describe a hypocritical or deceitful person who appears innocent or harmless but is actually malicious or cunning.
In the song, the singer laments that he gave his love to a "mosca muerta," implying that the person he loved betrayed him despite appearing sweet and innocent. This colorful expression adds a layer of intrigue and drama to the lyrics, making it a memorable and engaging word choice.
This song, "Este Borracho" by Banda MS, is a classic tale of heartbreak and regret, but with a really blunt and almost darkly humorous edge. Our singer is playing the part of a 'borracho' [drunk] who's dealing with the pain of a bad breakup. He feels like he's "pagando una condena" [paying a sentence] because of this person, and he's still obsessed with their "caricias, esa boquita" [caresses, that little mouth].
What's really interesting is how he swings between needing this person and wanting revenge. He admits he "necesita de tus besos, aunque sean falsos" [needs your kisses, even if they're fake] just to ease his pain. But then, he gets really intense, promising, "te haré que muerdas la tierra" [I'll make you bite the dust] once he can finally forget her. It's a pretty wild ride of emotions, from deep sadness to outright anger, all fueled by a broken heart and too many drinks.