Ağzımı açtırmasın literally translates to "don't make me open my mouth." It's an idiomatic expression that means "don't make me speak" or "don't make me say things I'll regret."
In the song, the singer is warning Sebastian to tell someone to stop spreading rumors, otherwise, the singer will reveal the truth or say something harsh. It's a common, yet impactful, phrase in Turkish that conveys a strong warning.
Have you ever been so over a breakup that your ex's drama just makes you laugh? That's the mood of this sassy Turkish pop hit. The singer, Hande Yener, hears that her ex is telling everyone he's leaving her and "burning his ships," a dramatic way of saying he's making a final decision. Her reaction? She finds it hilarious. She's also hearing rumors that she's the one who changed and grew distant, and she seems to agree, feeling completely justified.
Instead of getting upset, she sends a message through a mysterious figure named Sebastian. She warns her ex, "Tell him, Sebastian, not to make me open my mouth." She's holding back some truths that could hurt him and is telling him to stop talking behind her back. If he really has a problem, he should face her directly. The song became a huge hit, and "Tell him, Sebastian..." turned into a popular meme in Turkey for when you want to send a bold message to someone indirectly.