Say when Definition 1 of 1Pro Tip 1/3Politeness Marker Indicates you're being considerate by allowing them to control the amount they want. I'm adding sugar to your tea; say when. Pro Tip 2/3Common in Service Used by someone providing a service, like pouring wine or giving you a serving of food. Tell me when your plate is full enough. Pro Tip 3/3Requires Immediate Response You should respond as soon as you want the person to stop without delay. I'm grating cheese on your pasta—say when!