When to use actively fangirling or fanboying in Korean
Korean: 덕질[ deok-jil ]

When to use it
덕질 means being a very active fan of someone or something you love, like a singer, actor, character, or sports team. People use it when they watch videos, collect goods, go to events, or talk excitedly about their favorite thing. It is casual slang, so it is common in daily conversation but not very formal.
Example sentences
집에서 아이돌 영상 보면서 덕질해요.
I fangirl/fanboy at home while watching idol videos.
학교에서 친구랑 같이 그 배우 덕질해요.
I fangirl/fanboy over that actor with my friend at school.
주말에 콘서트에 가서 하루 종일 덕질했어요.
I spent all day fangirling/fanboying at the concert on the weekend.
문구점에서 좋아하는 캐릭터 스티커를 사며 덕질해요.
I fangirl/fanboy by buying stickers of my favorite character at the stationery store.
카페에서 친구랑 덕질 이야기만 했어요.
At the cafe, I only talked with my friend about fangirling/fanboying.
부산 여행 가서 좋아하는 팀 굿즈를 보며 덕질했어요.
On my trip to Busan, I fangirled/fanboyed while looking at goods for my favorite team.