cave in
cave in {v.}
1. To fall or collapse inward.
The mine caved in and crushed three miners.
Don't climb on that old roof. It might cave in.
2. {informal}
To weaken and be forced to give up.
The children begged their father to take them to the circus until he caved in.
After the atomic bomb, Japan caved in and the war ceased.
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Source: A Dictionary of American Idioms