Misterio translates to "mystery." While it might seem like a common word, its repeated use in this song, especially in the chorus, makes it central to the song's theme and very enticing.
Ñejo uses it to express his confusion and frustration about why the woman he desires isn't taking him seriously, despite all his efforts. He asks, "¿Cuál es el misterio?" (What's the mystery?), highlighting his bewilderment and drawing the listener into his quest to understand her aloofness. This makes it an engaging and memorable word in the context of the song.
Have you ever tried really hard to get someone's attention, but they just won't take you seriously? That's exactly what Ñejo is singing about in "Mistério"! He's super frustrated, asking, "¿Cuál es el misterio?" [What's the mystery?] because he's gone everywhere looking for this person and even says he's "bajado la luna, te regalo el mundo" [brought down the moon, given you the world] – but still, no luck!
He's tired of feeling like he's always chasing them, asking, "¿Me piensas tener gateando detrás de ti toda la vida?" [Do you plan to have me crawling behind you my whole life?]. He even jokes that if Cupid hasn't shown up by now, he's probably forgotten his bow and arrow! Ñejo is ready to make sacrifices, like quitting vices and exercising, but he wants some kind of benefit in return. He even warns her that other girls are watching and he might just find someone else "con las tetotas y las nalgotas más grandotas" [with bigger boobs and bigger butts] if she doesn't start taking him seriously. It's a funny, blunt, and very relatable take on the push and pull of trying to win someone over!