Pull the plug

To withdraw support and thereby terminate something dates from the early 20th century. Originally, this expression was in reference to 18th century flushing systems by which lavatories required the pulling of a stopper or plug to release the downward flow of water from cisterns that were then re-filled by hand. Pulling the plug removed all support for the water and from this the current figurative connotation developed. Pulling the plug is also recorded in the context of WWII RAF slang for opening the bomb bay and releasing the bombs.