Draw-a-line-in-the-sand Definition

verb

(intransitive) To lay down a challenge; to provide a test of commitment (often one which carries a high risk) to a cause.

Wiktionary

(idiomatic) To create a real or artificial boundary or distinction between (two places, people or things).

Wiktionary

(idiomatic) 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.

Wiktionary

Origin of Draw-a-line-in-the-sand

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.

From Wiktionary