On lists of nautical flag meanings ([one](http://www.sailorschoice.com/seamanship/alphabet_pennants.htm) and [another](http://www.maineharbors.com/flag.htm)), 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.