give somebody an inch
US /ɡɪv ˈsʌm.bɑː.di ən ɪntʃ/
UK /ɡɪv ˈsʌm.bɑː.di ən ɪntʃ/

1.
dar un dedo y tomarse la mano, aprovecharse de una pequeña concesión
to allow someone a small amount of freedom or advantage, which they then try to exploit to gain much more
:
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If you give him an inch, he'll take a mile, so be firm with your boundaries.
Si le das un dedo, se tomará la mano, así que sé firme con tus límites.
•
My boss is very strict because he knows if he gives us an inch, we'll slack off.
Mi jefe es muy estricto porque sabe que si nos da un dedo, nos relajaremos.