Line in the sand is an idiom, a metaphorical (sometimes literal) point beyond which no further advance will be accepted or made.
Related terms include unilateral boundary setting, red lines and ultimatums to define clear consequences if a line is crossed.
Some have (perhaps erroneously) interpreted Jesus' writing in the sand, as drawing a line in the sand in order to address those who are about to stone a woman caught in adultery.
The exact origin of the phrase is unknown: the Oxford English Dictionary suggests a transitional use from 1950, but a definitely figurative use only as late as 1978: He drew a line in the sand with the toe of his boot, and said, 'It's as though I told you "I can punch you in the nose, but you can't reach across that line to hit me back.
"'Notwithstanding the supposed public revulsion toward more federal spending, waste and bureaucracy-building, Congress seems to have gone out of its way to draw a wide line in the sand in front of Carter.There is reference to an actual line being drawn in several historical, or legendary, military events: