The common saying "give an inch and they'll take a mile" means:
- 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!
It is a very old saying which originally had a differen phrasing:
- 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. (Dictionary.com)
The same expression is common also in other languages where "the arm", unlike in English, is still part of the saying:
Donnez-leur en long comme le doigt, ils en prendront long comme le bras (French)
Dale un dedo y se toma hasta el codo (Spanish)
Dai una mano e si prendono un braccio (Italian)
What is the origin of the saying? Was it an original English one or was is "imported" from a foreign language?
Why was "ell" changed to "mile" (a very different measure) around the beginning of the 20th century?
1) - the assumption that ell has fallen completely out of use does not appear to be supported by available reference: ( Ngran "take a mile vs take an ell". )
2) - idiomatic expressions don't typically rely on their literal meaning, so the need to change from ell to another common measure doesn't appear to be the only possible reason for the change.