어쩔 수 없이 ~았/었어요 · 어쩔 수 없어요
with no choice, inevitably, against one's will
Example
버스가 끊겨서 어쩔 수 없이 택시를 탔어요.
The buses had stopped running, so I had no choice but to take a taxi.
Word parts
Literally 'without a way to do (어쩌다) anything'.
Good to know
As a predicate: 어쩔 수 없어요 'It can't be helped.'