Sometimes we tell about their origin story – where they come from . But origins are often a lot of guesswork . We often don't know how some expressions began . That is not the case with today's expression – to cross the Rubicon . But before we talk about its origin , let's talk about its meaning . To cross the Rubicon means to make a decision of permanence . It cannot be changed . It is irreversible - a done deal – no cancellations ! Someone who has crossed the Rubicon has reached a point where they cannot change a course of action . In other words , there is no going back . Now for its origin . This expression comes from a story about Julius Caesar . Rubicon is the name of the river he crossed with his army . Experts say this started a civil war in Rome in 49 BCE . When our enemies draw a line in the sand and we pass over that line , we could also say we have crossed the Rubicon . Therefore , the result or consequence of the decision or action is severe . It has weight . If I cross the Rubicon , I have made a fateful and final decision . Therefore , we don't use this expression for low-stakes situations – times when nothing is at stake . If nothing is at stake , nothing important is at risk . For those lighter times , we can say , that ship has sailed or that train has left the station . So , save cross the Rubicon for a heavy , meaningful decision or action . Here are two friends using the expression . They broke up . They seemed like such a great couple . Well , Angela accepted a dream job out in Alaska . She's going to be researching polar bears . That sounds great ! It is great . The problem is … she didn't tell Dimitry . He found out from some guy who works at their favorite coffee shop . Oh , that's not good . Dimitry said he can forgive a lot but not this . She crossed the Rubicon when she decided to move without telling him first . I like Angela .