出典:Wiktionary
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.
draw a line in the sand (三人称単数 現在形 draws a line in the sand, 現在分詞 drawing a line in the sand, 過去形 drew a line in the sand, 過去分詞 drawn a line in the sand)