6세가 되면 첫 영구치인 어금니가 나오는데 6세의 '6'과 영구치(어금니)의 '구'를 따온 것입니다.
When you turn 6, you get your first permanent molar, which is named after the 6-year-old's molar, which is 6 from 6 years-old, and molar from the molar.