Wikipedia says “a line in the sand” is a metaphor referring to a point beyond which no one can proceed, or an act with consequences that are permanent and irreversible. The article states the origin is unknown.
A suggested Biblical link seems like a stretch to me. (John 8:6 reads: This they said, testing Him, that they might have something of which to accuse Him. But Jesus stooped down and wrote on the ground with His finger, as though He did not hear. [NKJV] I don’t see the current meaning of “a line in the sand” here.)
Wikipedia lists instances of an “actual line being drawn in several historical, or legendary, military events” even if the phrase wasn’t used.
A National Geographic article says “one of the earliest recorded instances of anyone drawing a line in the sand took place in ancient Rome around 168 B.C.” when a Roman envoy “drew a line in the sand around [the opposing king] and told him he had to decide [if he would accede to Roman demands] before he crossed it.” Continue reading →