Desarma literally means "disarms" or "takes apart." It's a strong verb that conveys a sense of being completely undone or broken down.
In the song, the artist sings, "Pa' no sentir tu adiós que me desarma" (So I don't feel your goodbye that disarms me). Here, it's used metaphorically to express how the pain of a breakup leaves him feeling vulnerable and completely shattered, as if his emotional defenses have been stripped away.
Have you ever tried to convince yourself you're over someone, but your heart just isn't getting the memo? That's exactly what The La Planta is singing about in "Por El Contrario"! He's asking, "Si se supone que ya te olvidé... Dime, ¿Qué hago aquí en mi soledad pensando en cómo estás?" [If I'm supposed to have forgotten you... Tell me, what am I doing here in my loneliness thinking about how you are?]. It's that classic post-breakup struggle where you're trying to move on, but every quiet moment brings them back to mind.
He's really wrestling with the idea of his ex being with someone new, wondering "Si todavía sigo en tu cabeza" [If I'm still in your head] and if they've even covered up his name with a new tattoo! Meanwhile, he's doing the exact opposite, admitting, "Yo, por el contrario, casi pierdo el alma pa' no sentir tu adiós que me desarma" [I, on the contrary, almost lost my soul so as not to feel your goodbye that disarms me]. He even tried to negotiate with karma to avoid the pain, but karma told him, "Todo se paga, todo se paga" [Everything is paid for, everything is paid for]. It's a really honest look at how hard it is to let go, even when you know you should.