Desenfundaba comes from the verb desenfundar, meaning "to unsheathe" or "to draw a weapon from its holster." It’s a very vivid and dramatic word, like something straight out of an old Western movie.
In this corrido, the lyrics say that Ramón Arellano "Rápido desenfundaba" (He would quickly draw his gun) if anyone challenged him. The -aba ending is the imperfect tense, which signifies this was a habitual action, powerfully painting a picture of his ruthless and volatile character.
This song is a classic corrido, a story-song that tells you all about a real-life figure. In this case, it's Ramón Arellano, a man also known as 'El Colores'. The lyrics describe him as a 'Hombre de pocas palabras' (Man of few words) who was the 'Orgullo de sus hermanos' (Pride of his brothers). He was so feared that even though his voice wasn't deep, it 'Ponía a temblar a cualquiera' (Made anyone tremble). He was not someone you wanted to cross.
The song lays out exactly why he was so intimidating. He carried an AK-47, a pistol, and 'Varias granadas de mano / Listas para detonarlas' (Several hand grenades / Ready to be detonated). He controlled the border 'a punta de balas' (at gunpoint). The lyrics are very direct about his methods: if you complained, he would quickly draw his gun and 'Les disparaba en la cara' (Shoot them in the face) without saying a word. The song ends by comparing him to a Sicilian mobster, saying he 'ajustaba las cuentas / Siempre al estilo italiano' (settled scores / Always in the Italian style).