To quickly, unexpectedly, or abruptly do something without explanation. Used especially with "leave" and related verbs, though not exclusively. I must have offended him, because halfway through the conversation he just up and left. The town has been in a panic searching for a young girl who up and vanished over the weekend. Everyone asks how I came up with the story, thinking it must have taken all this time and planning, but the truth is one day I thought up the premise and then I just up and wrote it.