mulac

1. Croat. colloq. derog. (of a child, teenager) brat; young hooligan
   Neki mali mulac mi je razbio prozor. — Some little brat broke my window.
2. Croat. colloq. derog. (of an adult, youth) immature person; overgrown kid
   Ne budi mulac, razmisli malo. — Don't be such a kid, think a little.

In Serbia a brat is derište or mangup, and an immature, unserious person is balavac or klinac.