give an inch and they'll take a mile

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.
See also: and, give, inch, mile, take, will

give an inch and they'll take a mile

Make a small concession and they'll take advantage of you. For example, I told her she could borrow the car for one day and she's been gone a week-give an inch! This expression, in slightly different form, was already a proverb in John Heywood's 1546 collection, "Give him an inch and he'll take an ell," and is so well known it is often shortened (as in the example). The use of mile dates from about 1900.
See also: and, give, inch, mile, take