Rubicon a stream in NE Italy which marked the ancient boundary between Italy and Cisalpine Gaul. Julius Caesar led his army across it into Italy in 49 bc, breaking the law forbidding a general to lead an army out of his province, and so committing himself to war against the Senate and Pompey; the ensuing civil war resulted in victory for Caesar after three years.

In general use, rubicon came in the 17th century to mean a boundary or limit, and from the late 19th century, in the card-game of piquet, to denote an act of winning a game against an opponent whose total score is less than 100, in which case the loser's score is added to rather than subtracted from the winner's.
cross the Rubicon pass a point of no return, as Caesar led his army across the river forming the ancient boundary between Italy and Cisalpine Gaul; the expression has been current since the 17th century.

Ru·bi·con / ˈroōbəˌkän/ a stream in northeastern Italy that marked the ancient boundary between Italy and Cisalpine Gaul. Julius Caesar led his army across it into Italy in 49 bc, breaking the law forbidding a general to lead an army out of his province, and so committing himself to war against the Senate and Pompey. The ensuing civil war resulted in victory for Caesar after three years. ∎  [as n.] a point of no return: on the way to political union we are now crossing the Rubicon.

ru·bi·con • n. (in piquet) an act of winning a game against an opponent whose total score is less than 100, in which case the loser's score is added to rather than subtracted from the winner's.• v. (-coned, -con·ing) [tr.] score a rubicon against (one's opponent).

Rubicon Ancient name for the River Fiumicino in n central Italy. It formed the border between Italy and Cisalpine Gaul. In 49 bc, Julius Caesar precipitated civil war by ‘crossing the Rubicon’ into Italy with his army, hence the modern phrase meaning to take an irrevocable step.