kletva

1. curse
   Baba je na njega bacila kletvu. — The old woman put a curse on him.
   Kažu da nad tom kućom visi kletva. — They say there's a curse on that house.
2. dated oath
   Održao je kletvu datu ocu. — He kept the oath he gave his father.

For "oath" modern Serbian usually uses zakletva.