( saying ) used to say that if you allow some people a small amount of freedom or power they will see you as weak and try to take a lot more
GIVE SB AN INCH (AND THEY'LL TAKE A MILE / YARD)
Meaning of GIVE SB AN INCH (AND THEY'LL TAKE A MILE / YARD) in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005