Mentías is the imperfect past tense of the verb mentir (to lie). It translates to "you used to lie" or "you were lying."
In the song, the singer repeatedly uses this word to express the pain and betrayal he feels after realizing his lover's promises of love were false. It's a direct and emotionally charged word that captures the central theme of deception and heartbreak in the lyrics.
Have you ever been so in love that you just couldn't see the truth, even when it was right in front of you? That's exactly where the singer of Los Angeles de Charly's "Mentías" finds himself. He's driving down the highway, and every love song on the radio just reminds him of the person who broke his heart. He can't get her out of his head, even though he desperately wants to erase her name from his heart.
He remembers her kisses, calling them a "trampa como libro abierto" [trap like an open book], and how she swore she loved him. He was so blind with love, or "ciego por ti" [blind for you], that he believed all her lies. She told him so many things, "palabras que caían como lluvia" [words that fell like rain], and he was completely smitten. He feels like he was just her "juguete preferido en tu vida" [favorite toy in your life]. But even with all the deception, he still says, "Pero no me arrepiento haberte amado" [But I don't regret having loved you]. It's a really relatable feeling of loving someone so much, even when they hurt you.