There have been many explanations for where this phrase comes
from. SoMost of them are obviN%=yously spook etymology, but the first one I
heard -- and who knows if this is right -- comes from WW I, where just
before one side would go into battle, they'd send one man up in a hot
air balloon to scout the enemy's location. If you saw the balloon
going up, you knew combat wasn't far behind.
Wherever it comes from, though, that's the general meaning of