Pendejo is a strong, informal, and somewhat vulgar term that translates to "idiot" or "asshole." It's used to express contempt or frustration.
In this song, the singer, still clearly hurt and jealous, offers to explain to the new partner "la forma correcta de hacerte el amor" (the correct way to make love to you), referring to him as "el pendejo." This word choice highlights the singer's lingering anger and sense of superiority, making it a very impactful and memorable part of the lyrics.
This song is a wild ride of emotions after a breakup! Gerardo Coronel sings about wishing his ex the best, even hoping her new partner loves her "mucho, mucho más que yo" [much, much more than I did]. But then things take a hilarious and slightly unhinged turn.
He offers to call the new guy and "explicarle paso a paso cómo elevar tu pasión" [explain step by step how to elevate your passion], and even "decirle al pendejo la forma correcta de hacerte el amor" [tell the idiot the correct way to make love to you]. Talk about a backhanded compliment! Despite all this, he tells her to delete his number because he knows he'll keep calling her. He just wants her friends to keep him updated on her life. It's a mix of genuine well-wishes, lingering bitterness, and a whole lot of dramatic flair.