Desarmaste comes from the verb desarmar, meaning "to disarm." It's an interesting word because it's not commonly used in everyday romantic contexts, making it stand out.
In the song, the singer uses it metaphorically: "Con tu poder me desarmaste" (With your power, you disarmed me). This implies that the person's arrival completely caught him off guard and broke down his defenses, despite his initial desire to remain emotionally distant. It vividly conveys the unexpected and overwhelming impact of love.
Have you ever been so annoyed by falling in love that you almost... hate it? That's exactly what Reik, a band from Mexico, sings about in "Te Odio" [I Hate You]. Our singer was perfectly happy wanting his space, minding his own business, and then bam! Someone came along and completely disarmed him.
He literally says, "Ando enamorado y yo quería mi espacio" [I'm in love and I wanted my space]. He didn't want anything serious, but he fell anyway, calling himself a "pendejazo" [idiot] for it! It's a funny, relatable take on suddenly being head-over-heels when you least expect it. He's so smitten that even listening to her heart is like a "gran canción" [great song] that will accompany him for life. He might say "Te odio" [I hate you], but it's clear he's completely, wonderfully, annoyingly in love.