draw a line in the sand

Meanings

1. To lay down a challenge; to provide a test of commitment (often one which carries a high risk) to a cause.
2. To create a real or artificial boundary or distinction between (two places, people or things).
3. To indicate the threshold or level above which something will become unacceptable or will provoke a response; to create a boundary and imply or declare that its crossing will provoke a (negative) response.

Etymology

Unknown. In the US, it is commonly accepted as a reference to the action of William B. Travis, who, in 1836, while commanding the defenders of the Alamo and contemplating a demand for surrender, drew a line in the sand and asked those willing to remain and defend the Alamo to their deaths to step across.