Pendejo is a strong, informal word that can mean 'idiot' or 'dumbass'. While it's often used as an insult, it can also imply someone is naive or inexperienced.
In this song, Humbe uses it to assert his maturity and street smarts, singing, "Puedo estar joven pero no pendejo" (I might be young but I'm not dumb/naive). It's an engaging word that adds a defiant edge to his message.
Humbe's "EL POETA" is a really cool look into the mind of a young artist who's just starting to find his voice. He's got so much to say, even more than what's already in his songs! He sings, "Tengo mucho que decir, Tanta mierda que escribir" [I have a lot to say, So much shit to write]. It's like he's bursting with ideas and feelings, and he's not afraid to be blunt about it.
He's also got this rebellious streak, telling people to back off with "Con permiso yo los dejo, Puedo estar joven pero no pendejo" [With your permission I'm leaving you, I might be young but I'm not stupid]. It's clear he's on his own path, not chasing money or trying to impress anyone. He just wants to sing his words, even if he doesn't know the "right" ones. He even mentions composing with angels and dancing in the clouds, which is such a vivid image of his creative process. This song is all about the joy of creating, even when it's tough, and finding happiness in being "el poeta" [the poet] writing in his bed.