Juetazo is a colloquial and somewhat aggressive term that literally means 'whiplash' or a 'hard smack'. It's not a word you hear every day, making it quite memorable.
In the context of the song, the singer uses it to threaten her on-again, off-again lover, saying, "Que venga para que tu sientas tu juetazo" (Let him come so you can feel your whiplash/smack). It implies a forceful and impactful lesson, suggesting she's moving on to someone who will make her ex regret his actions.
"Lunes a Jueves" is about a partner who only shows up when it's convenient. From Monday to Thursday he vanishes without a word, then calls her back the moment the weekend starts, expecting things to pick up right where they left off.
Leslie Grace sings about being done waiting around for someone who treats her like an afterthought. She reminds him what he's about to lose, and by the end she's already looking past him, ready to let someone else enjoy what he took for granted.
It's a song about self-respect: refusing to be someone's part-time plan, and walking away with your head high instead of waiting by the phone.