Definition of give someone an inch and he (or she) will take a mile in US English:

give someone an inch and he (or she) will take a mile

phrase

proverb
  • Once concessions have been made to someone they will demand a great deal.

    • ‘You give her an inch, she'll take a mile. Next thing you know, she's expecting you to clean up after yourself, pick out your own clothes for the day, and even flush the toilet.’
    • ‘And he's proven in the past that, if you give him an inch, he'll take a mile.’
    • ‘Never lend Mike any money, he is something of a kind that you give him an inch and he will take a mile.’