from Wiktionary, Creative Commons Attribution/Share-Alike License
- interj. Used by servants in medieval Scotland to warn passers-by of waste about to be thrown from a window into the street below. The phrase was still in use as late the 1930s and '40s, when many people had no indoor toilets.
from the GNU version of the Collaborative International Dictionary of English
- n. An old cry in throwing water, slops, etc., from the windows in Edingburgh.
from The Century Dictionary and Cyclopedia
- Look out for the water: a cry formerly used in Edinburgh, Scotland, to warn passengers to beware of slops about to be thrown out of the window.
French garde à l'eau translated means "beware of the water." (Wiktionary)