like a dog with a bone
US /laɪk ə dɔɡ wɪθ ə boʊn/
UK /laɪk ə dɔɡ wɪθ ə boʊn/
1.
to be very determined and refuse to give up, especially when trying to get information or achieve something
:
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She went after that story like a dog with a bone, digging up every detail.
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Once he gets an idea like a dog with a bone, he won't let it go.