put one's foot down

To be unyielding or inflexible in one's position or decision. The kids complained and complained when we refused to get a puppy, but we had to put our foot down. As a manager, you have to put your foot down sometimes, or your staff will walk all over you.

Take a firm stand, as in She put her foot down and said we could not go to the carnival. This idiom alludes to setting down one's foot firmly, representing a firm position. [Late 1800s]

To take a firm position. The analogy presumably is to setting one or both feet in a fixed position, representing a firm stand. Although versions of this term (usually with set one’s foot down) exist from the sixteenth century on, it became current only in the nineteenth century. The OED cites James Payn’s The Luck of the Darrells (1886): “She put her foot down . . . upon the least symptoms of an unpleasantry.”