noun
wasted effort; fruitless effort; vain efforts
Refers to effort that yields no result; often used in the phrase 無駄骨を折る (to waste one's efforts).
彼の努力は結局無駄骨に終わった。
His efforts ended up being wasted.
そんなことをしても無駄骨を折るだけだ。
Doing that will only be a waste of effort.
Compound of 無駄 (muda, 'waste') and 骨 (hone, 'bone'), literally 'waste bone', figuratively meaning wasted effort.