When to use romantic something brewing in Korean
Korean: 썸[ sseom ]

When to use it
썸 means two people are starting to like each other in a romantic way, but they are not officially dating yet. It is a common slang word for that in-between stage when something is slowly growing. People use it when talking about a new crush or a possible relationship.
Example sentences
나 요즘 같은 반 친구랑 썸 타요.
These days, I have something going on with a classmate.
집에서 엄마가 내 썸 이야기를 물었어요.
At home, my mom asked about my little romance.
공원에서 둘이 자주 만나서 썸 같아요.
They meet often in the park, so it seems like something is going on.
너 민지하고 썸이야?
Are you and Minji kind of seeing each other?
쇼핑몰에서 썸한테 줄 모자를 샀어요.
I bought a hat for the person I am kind of seeing at the mall.
여행에서 만난 사람과 썸이 생겼어요.
I started a little romance with someone I met on a trip.