used in Edinburgh as a warning cry when it was customary to throw slops from the windows into the streets
"Residents often threw refuse out of windows at night onto the streets. A commentator observed that, 'One never knew the moment when the warning cry 'Gardyloo'... might ring out, following which would come in quick succession an avalanche of unmentionable filth on to the footpath – or the passer-by.'" — Jonathan Yeager, Enlightened Evangelicalism: The Life and Thought of John Erskine, 2011
About the Word:
Pity the visitor to Scotland unfamiliar with the practice of using what is most likely a French-based term (garde à l'eau! literally means "look out for the water!") when dumping slops into the streets.