엎친 데 덮친 격으로 · 엎친 데 덮친 격이에요
to make matters worse; one misfortune on top of another; when it rains, it pours
Example
지갑을 잃어버렸는데 엎친 데 덮친 격으로 비까지 왔어요.
I lost my wallet, and to make matters worse, it started raining too.
Word parts
격 manner
Literally 'like being pounced on when you've already fallen.'
Good to know
In formal writing, 설상가상(으로) means the same thing.