give (someone) an inch and they’ll take a mile

Yield only a little and you’ll be taken advantage of. This expression began life as a proverb, “Give him an inch and he’ll take an ell,” cited in Heywood’s 1546 collection. Around the turn of the twentieth century mile entered the picture, as in W. D. Steele’s The ManWho Saw Through Heaven (1927): “Give these old fellows an inch and they’ll take a mile.”