无力回天 is a four-character idiom (成语) meaning 'to be powerless to reverse an irreversible outcome' or 'to be unable to turn the tide'. The literal image is 'no strength to turn the sky', but the figurative use is about facing a hopeless situation where no effort can change the result. It is typically used as a predicate in formal or written Chinese, often in contexts of failure, loss, or inevitable decline.