Pendejo is a very common, yet strong, slang term in Spanish that can mean 'idiot', 'asshole', or 'fool'. While it's often considered vulgar, it's widely used in informal contexts and music.
In this song, the singer uses it to describe himself, saying "Soy un pendejo por creer tus mentiras" (I'm an idiot for believing your lies). This self-deprecating use makes the word particularly impactful, conveying deep regret and self-blame after being betrayed.
"Soy Un P*endejo" is a heartbreak song from Eslabon Armado, the Regional Mexican group known for pairing sierreño guitars with raw, confessional lyrics. Over the song the singer confronts a partner who has been quietly seeing someone else, asking her directly why she is leaving him for another man when he was right there for her, and why she went quiet the moment he asked what happened.
The title, which roughly means "I'm an idiot," is aimed at himself as much as at her. He recalls giving her his trust and defending her to friends who warned him something was wrong, only to realize he was the last to know. By the chorus he keeps repeating "Soy un pendejo" (I'm a fool) for believing her lies while she was out at night with "friends" that were never really there, and the song ends in bitter resignation, with the singer left with nothing and wishing the same pain on her, "a thousand times worse."