Achicopale' is a colloquial and somewhat rare form of achicopalarse, meaning "to get discouraged" or "to feel small/intimidated". It's a fun, informal word that adds character to the lyrics.
In the song, Arcángel uses it to reassure his love interest, saying, "No te achicopale', que ya hace tiempo que nos conocemos" (Don't get discouraged, we've known each other for a while now). It's an endearing way to tell someone to relax and be comfortable, especially in a flirtatious context.
In "Misterio," Arcángel is completely captivated by a woman on the dance floor. He finds everything about her fascinating, but what truly mesmerizes him is how she moves. He describes her dancing as a "misterio" (a mystery), something so powerful and alluring that you have to take it "en serio" (seriously). Her presence is so intense that it's almost intimidating, or as he puts it, it reaches a level that "asusta" (scares).
The song uses vivid imagery to describe her dance moves. Arcángel compares her to a "serpiente" (snake), whose "veneno" (venom) he adores, and also to an "águila" (eagle), soaring high. The lyrics are a direct expression of his intense attraction, inviting her to leave with him without looking back. The repeated word "bellaca" is Puerto Rican slang that emphasizes her confident and provocative energy, which the singer clearly finds irresistible.