썰다 [sul da] = to cut, to chop (with a knife)

어떤 사람들은 양파를 썰 때 눈물을 흘려요.

uh tun sa ram deul eun yang pa reul sul te noon mool eul heul lyuh yo.

= Cutting an onion makes some people cry.

https://www.learnwitholiver.com/korean/daily-sentence-1736