to make matters worse; besides; what's more; in addition; on top of that
雨が降り、おまけに風も強い。
It's raining, and to top it off, windy.
遅刻して、おまけに忘れ物もした。
I was late, and I forgot something too.
高いし、おまけに美味しくない。
It's expensive and not even tasty.