give someone an inch
give (one) an inch and (one) will take a mile
If one makes concessions for someone, that will embolden that person to take further advantage of one, instead of being content with what they have been given. If you let the kids stay up later on the weekends, they'll want to do it all the time. Give them an inch and they'll take a mile.
give someone an inchonce concessions have been made to someone they will demand a great deal.
The full form of the saying is the proverb give someone an inch and he will take a mile . In former times, ell (an obsolete measure of length equal to a little over a metre) was sometimes substitued for mile .