Chompa is a slang term, particularly used in some Latin American countries, for a beanie or knit cap. While it can also refer to a jacket in other contexts, in this song, with the phrase "De la chompa rapada", it clearly refers to a beanie worn by someone with a shaved head.
It's an interesting word because it's not the most common term for a beanie (gorro is more standard), making it a unique and authentic piece of street slang that gives insight into the specific cultural context of the song's barrio setting.
Santa Fe Klan's "15 DE SEPTIEMBRE" is a high-energy anthem all about loyalty to his neighborhood and his crew. He's from the Santa Fe neighborhood in Monterrey, Mexico, and he makes it clear that no matter where life takes him, he wants to return to where he grew up: "Si mi vida termina lejos de casa, me quiero ir / Al lugar donde niño crecí" [If my life ends far from home, I want to go / To the place where I grew up as a child].
This song is a full-on party vibe, celebrating life in the barrio with his friends. He talks about shaved heads, tattoos, and cold beer: "De la chompa rapada, la piel tatuada / Cerveza helada, andamos de parranda" [Shaved heads, tattooed skin / Cold beer, we're out partying]. There's a strong sense of community and pride, with shout-outs to different neighborhoods and cities across Mexico, all while keeping the party going "entre tragos y fumadas" [between drinks and smokes]. He also touches on the darker side of barrio life, mentioning police harassment and violence, but always circling back to the importance of his home and his people. It's a powerful declaration of identity and belonging.