성에 차지 않아 is a fantastic idiomatic expression meaning 'not satisfied' or 'not enough to fill me up.' It's not your basic 'I want more'—it has a nuanced, almost poetic flavor that's rare in everyday conversation.
In the lyrics, after singing about drinking and drinking but still feeling empty, this phrase captures that deep, unfulfilled craving. It's the perfect setup for the song's central metaphor, where the beloved becomes a soda pop that finally quenches that thirst.
This song is one big, fun metaphor for being completely obsessed with someone. Saja Boys compares the person he desires to a soda pop, and he is thirsty. He feels empty and needs them to "fill me up." He says that even after drinking and drinking, "성에 차지 않아" [I'm not satisfied], which tells you just how addictive this person is to him.
He's not subtle about it either! He calls them a snack, saying, "Take a big bite, want another bite, yeah." The whole song builds on this idea of wanting more and more, with lines like "Pour me up, I won't stop" and "I need a second round." It's a super playful and direct way to describe an intense crush, where this one person is so refreshing and exciting that everyone else seems boring in comparison: "너 말곤 모두 뻔해, 뻔해, 뻔해" [Everyone but you is boring, boring, boring].