To commit to a particular plan or course of action. The phrase refers to how Julius Caesar crossed the Rubicon river and became embroiled in civil war in 49 BCE. Look, if you cheat on this test, you are passing the Rubicon, man. You can't take that back. I think I passed the Rubicon when I took this management position. It would be a huge pay cut to go back to my old job, and my boss would be furious.