Pucheros literally refers to a pout or a sulky expression, often made by children. It's a charmingly expressive word that paints a vivid picture of someone feeling down.
In this song, the singer admits, "Hice pucheros con los primeros tequilas que me tomé" (I pouted with the first tequilas I drank). This line humorously conveys his sadness and self-pity after a breakup, showing a vulnerable and relatable side to his heartbreak.
Espinoza Paz turns a Sunday breakdown into a clock. He starts by hiring a band, "Jalé la banda" [I hired the band], to play him a set of songs about her, then just narrates the hours as they wear him down: ten o'clock passes, and by eleven he's cursing her name again, "Dieron las once y te maldije otra vez" [Eleven o'clock hit and I cursed you again]. Midnight is where the song gets its title, "Como a las doce, perdí la pose" [Around twelve, I lost my cool], one guy watching his own composure fail on schedule.
By three he's passed out cold, "De dos a tres quedé inconsciente y me dormí" [From two to three I passed out and slept], and four o'clock brings the tears back along with a mean streak, "Y me puse descortés" [And I got rude]. He explains the whole exercise near the end: "No canto por que sé compadre" [I don't sing 'cause I know how, buddy], "Canto para desahogarme" [I sing to vent]. Writing a breakup like a bar tab, hour by hour, is a strange choice, but it is exactly why this one feels so lived-in.