Calaña is a powerful word that translates to "ilk," "sort," or "character," and it's almost always used negatively to describe someone of low quality or bad moral fiber. It's not a word you hear every day, making it a fantastic term for expressing strong disdain.
In the song, Juanes sings that he has finally realized her "mala calaña" (her bad character). He uses this sharp, almost old-fashioned insult to show he now sees her for the bad sort of person she truly is.
Colombian rocker Juanes delivers the ultimate breakup diss track in "Mala Gente" (Bad Person). The story flips the script on a cruel ex. At first, she dumps him right when he needs her most and tries to offer the dreaded friend zone with "Tú me pides que seamos solo amigos" (You ask me that we just be friends). Juanes shuts that down instantly, responding that he has zero interest in being buddies after she mocked his feelings. But then the tables turn: she comes crawling back asking for another chance ("Tú me pides que regrese ahora" / You ask me to come back now), only to find that he has finally seen through her deception and moved on.
He does not hold back his anger, calling her "traicionera" (treacherous/traitor) and a "mentirosa" (liar). He even calls out her "mala calaña", an idiom that means having bad character or being of poor stock. The dramatic climax comes when he gleefully tells her "¡Y en el infierno enterita, enterita te vas a quemar!" (And in hell completely, completely you are going to burn!). It is petty, fiery, and wildly fun to sing along to when you are completely over an ex.