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 kicks off this track by admitting he's completely stuck in the past. He tells us, "Que ya ha pasado tanto tiempo y aún vivo el ayer" (That so much time has passed and I still live in yesterday). He's been waiting for someone who isn't coming back, and he's tired of getting his hopes up. He asks himself why he even still thinks about her, especially when she told him she wanted this "costal de huesos" (bag of bones) but it was all a "mentira como tus besos" (lie like your kisses). It's a really direct and self-aware way to describe feeling used.
He then gets lost in a fantasy, wishing he could somehow make her "loca por mí y te mueras de deseo de conmigo estar" (crazy for me and dying with desire to be with me). But he immediately snaps out of it, admitting, "Pero eso sólo lo pienso" (But I only think that). The reality is much simpler and sadder. What he really thinks about are "las noches que hacíamos el amor" (the nights we made love). He knows he can't change her and that trusting her blindly led to a huge fall from "la cima" (the top). It's a classic story of heartbreak, told with that signature Corrido Tumbado honesty.