Desatino translates to "folly" or "absurdity." It's a less common word that adds a touch of dramatic flair to the lyrics.
In the song, the singer uses it when he says, "Y aunque pueda parecerte un desatino, no quisiera yo morirme sin tener algo contigo." He's acknowledging that his intense desire to be with the person might seem irrational or foolish to them, but he's willing to embrace that perceived absurdity because his feelings are so strong. It highlights the depth of his longing in a unique way.
Have you ever been so into someone that being "just friends" feels like actual torture? That's exactly what Fidel Rueda is singing about in "Algo Contigo"! He's basically telling this person, "no hace falta que te digan que te muero por tener algo contigo" [you don't need to be told that I'm dying to have something with you]. It's pretty clear he's past the point of subtle hints.
He's struggling big time with the whole friendship thing, admitting "me cuesta ser tu amigo" [it's hard for me to be your friend]. He can't even get close to them without feeling a "manera loca" [crazy way] about them. He's even confessing to some light stalking, saying "ya no puedo continuar espiando dÃa y noche tu llegar" [I can't keep spying day and night on your arrival]. It's a funny, a little bit desperate, and totally relatable look at wanting to be more than just friends with someone you really like. He doesn't want to die "sin tener algo contigo" [without having something with you], which is a pretty blunt way to say he's all in!