Amargó is a form of the verb amargar, meaning 'to make bitter' or 'to sour'. It's a dramatic and poetic way to describe how someone can ruin an experience or even your life.
In the song, the singer exclaims, "Él me amargó la vida" (He embittered my life). She uses this powerful verb to convey the deep, lasting pain and resentment caused by her ex-partner's lies and mistreatment, turning her whole world sour.
Pimpinela are famous for their musical arguments, and this song is a perfect example of their dramatic style. It's a conversation between two people. One person is trying to play peacemaker for a man who wants a second chance, saying things like, "Él está desesperado" (He is desperate) and "Ese hombre aún te quiere" (That man still loves you).
But the other person is not interested at all. She shuts it down completely, listing all the terrible things this man did to her. She says he treated her like a beggar, cheated on her with anyone, and even "Me tenía prisionera" (He kept me a prisoner). Her final answer is blunt and powerful: "Ya no quiero oír su nombre" (I don't want to hear his name anymore) because "Ese hombre es mi enemigo" (That man is my enemy).