My daughter keeps saying "bung it" these days and she said jei jei told her it's a secret. From what my daughter told me, jei jei will say it when my daughter is bad. may be it means "forgive me" or "go away".
Does it pronounced closer to "bung niet"? Then it means ugly. Mine helper said that her son will say that if he did not like sth. So, luckily it is not a dirty word after all.