Say when is primarily an American idiom that has been in use since the 1800s. We will examine the definition of say when, where it came from and some examples of its use in sentences.
Say when is a phrase someone utters when pouring a drink for someone else, or pouring a liquid such as cream or syrup upon someone else’s food, or adding an ingredient such as Parmesan cheese or pepper to a dish at the table. Say when is short for say when to stop pouring. Say when is sometimes used by hosts or hostesses who are serving guests, by parents who are attempting to control their children’s consumption of sugar or other foods, and is often used by waiters. The correct response when one has been asked to say when may be either “stop”, “that’s enough”, “okay”, or to actually answer when. The answer when, used to answer say when, most probably began as a rather weak joke, but has now become an accepted response. The idiom say when was first used in the 1880s when pouring alcoholic beverages. While the idiom is still used when pouring alcoholic beverages, it is also used in any circumstance where one is serving food, drink or a garnish to someone else.
They came without chocolate, which confused me until two waiters with a golden goblet and ladle came over and, from a great height, started pouring, waiting for me to say “When”. (The Guardian)
As a grown man, the only time I can ever remember having to ‘say when’ is when you’re regulating a friend pouring alcohol. (The Manchester Evening News)