Costal de huesos literally translates to "sack of bones". It's a stark, informal phrase used to describe a very thin person.
In this heartbreaking song, Junior H uses it as a self-deprecating metaphor. When he sings, "Querías este costal de huesos" (You wanted this sack of bones), he's expressing the feeling that his lover only wanted him physically, reducing him to an empty shell. This raw, emotional phrase is a powerful example of the poetic and direct language in modern regional Mexican music.
Junior H brings his signature melancholic corrido tumbado sound to "LAS NOCHES", delivering an honest confession about being completely unable to move on. He opens by admitting he is still stuck in the past: "ya ha pasado tanto tiempo y aún vivo el ayer" ("so much time has passed and I still live in yesterday"). Even though he is tired of waiting, his brain refuses to quit fantasizing about a comeback. He literally calls himself "este costal de huesos" ("this bag of bones"), a self-deprecating Mexican idiom showing how worn down and fragile he feels after realizing all her sweet talk was "mentira como tus besos" ("a lie like your kisses").
The whole track feels like an internal debate with his own pride. He admits he ignored all the red flags with "tapándome los ojos, confié en ti" ("covering my eyes, I trusted you"), which sent him tumbling straight down from the peak: "me tenías en la cima, desde arriba fui a caer" ("you had me at the top, from way up high I fell"). He knows trying to change her mind is impossible, yet whenever the sun goes down, his thoughts wander right back to "las noches que hacíamos el amor" ("the nights we made love"). It is blunt, petty, and painfully relatable late-night overthinking set to acoustic guitar chords.