What is a maehyoong? I googled it, but it couldn't find anything.

Iirc it’s like a talisman that keeps the ghost tied to a place. Like the three ghosts that were haunting CP before ML took care of them. I think. Either way it’s something to do with more ghosts

Thanks for the explanation I was also so confused lol

Thanks.

You can try with 매흉 ;)
It means smth "ugly". Also one can bury it to curse someone, like making them sick or die (or both I belive).

The explanation has been given in Ch.13. Here's what it reads:

Maehyoong: burying impure objects in strategic places, cursing the victim to sicken or die.