얼쑤 (eolssu) is a traditional Korean interjection called a chuimsae (추임새). It doesn't have a direct one-to-one translation, but it's used like shouting "Hooray!", "Alright!", or "Let's go!" to express excitement and encourage performers, especially in traditional music like pansori.
In "IDOL", BTS proudly blends these traditional sounds into a modern K-pop track. Using 얼쑤 is a powerful statement celebrating their Korean heritage, perfectly matching the song's core message of unapologetic self-love and pride.
BTS's "IDOL" is a loud and proud celebration of loving yourself, no matter what anyone else thinks. The band directly confronts their critics from the very first lines, saying you can call them an artist or an idol, and they don't care which. As they put it, "아님 어떤 다른 / 뭐라 해도 / I'm proud of it" (or whatever else you call me, I'm proud of it). They brush off any hate by saying, "나는 전혀 신경 쓰지 않네" (I don't care at all), because they know exactly who they are and what they want.
The song's main message is shouted in the chorus: "You can't stop me lovin' myself." It's a powerful statement of self-acceptance. BTS also mixes in traditional Korean exclamations like "얼쑤 좋다" (Hooray, it's great!) and "지화자 좋다" (Yeah, it's great!), proudly blending their cultural identity with their global sound. They acknowledge that they have many different sides to their personalities—"내 속 안엔 / 몇십 몇백 명의 내가 있어" (Inside me there are tens, hundreds of mes)—but they love every single one.