Desarmadas literally means "disarmed" or "unarmed". While it can refer to taking away weapons, in this song, it's used metaphorically in a romantic context.
The lyrics say, "cuál sería el motivo de que me desarmadas de por vida con caricias y besos" (what would be the reason you disarmed me for life with caresses and kisses). Here, it means to be completely captivated and rendered defenseless by someone's affection, making it a unique and evocative choice for a love song.
Have you ever felt like love just happened to you, completely out of your control? That's exactly what La Zenda Norteña sings about in "Cual Seria El Motivo" [What Would Be The Reason]. The singer is totally bewildered, asking "cuál sería el motivo de que apareciera en mi vida la experiencia en el amor" [what would be the reason that the experience of love appeared in my life]. It's like love walked in, opened the door wide, and didn't even ask permission! He admits, "no puedo evitarlo estoy perdido" [I can't avoid it, I'm lost].
This song is all about being completely swept away. The singer feels permanently marked by this person, saying "me marcaste de por vida con caricias y besos" [you marked me for life with caresses and kisses]. He's so entangled, he feels like his body is caught and he's "hundido" [sunk]. He even says, "hoy no todos son ya un latido" [today not all are already a heartbeat], suggesting his very existence is now tied to this new love. He didn't go looking for it, but this person ended up being "todo lo que necesito" [everything I need], from the passion to the calls and messages he's now used to.