On lists of nautical flag meanings (one and another), the letter U means you are standing into danger. I am familiar with the phrase from a sailing point of view - it means something like "if you keep doing what you're doing, something bad will happen." Like if you're headed for a reef or something. Today I used it (in a gentle, teasing tone) to a family member who was annoying me. And they asked "why standing and not heading into danger?"
I think it's because it also applies if you stay still; it just means whatever you are about to encounter will not be good. But I'd appreciate a little background on the history and etymology of the phrase, and whether it applies only if you're moving, or not moving, or whatever.