Mentado is an interesting word that can mean 'well-known' or 'famous', but often carries a connotation of being 'infamous' or 'notorious', especially in certain contexts.
In "El Hijo Mayor," the lyric "Yo fui el hijo mayor de aquel señor tan mentado" translates to "I was the eldest son of that well-known/infamous man." The word hints at the father's reputation, suggesting he was a powerful figure, possibly involved in illicit activities, making it a key word for understanding the song's narrative and the family's background.
This song is a corrido, a type of Mexican ballad that tells a story, and it's a heavy one. The narrator is remembering a powerful figure who is now gone. You don't see his black Camaro speeding through the streets anymore, as he says, "Ya no se mira pasar / Ese Camaro negro" (You don't see that black Camaro passing by anymore). He describes a man who was "acelerado, atrabancado y muy valiente" (fast-paced, reckless, and very brave), someone who lived a dangerous life in the mountains, always armed and ready for trouble. He was an adrenaline junkie surrounded by a crew of "puro suicida" (pure suicidal types) who were always ready for a fight.
The whole song builds to a huge reveal. The narrator says, "A estas alturas ya sabrán de quién les hablo / Su nombre no puedo ni mencionarlo" (By now you'll know who I'm talking about / I can't even mention his name). Then he drops the title line: "Yo fui el hijo mayor de aquel señor tan mentado" (I was the eldest son of that well-known man). He's the son of a famous and feared figure, telling the story of his father's life and sudden end, because as he says, "así es la vida, de repente se nos va" (that's life, suddenly it's gone).