The phrase "add on" refers to the action of including or attaching something additional to an existing item, system, or situation. It often implies an increase or enhancement to what is already present, whether it's an extra feature to a product, a supplementary thought in a conversation, or additional costs in a financial context.
The phrase "add on" is derived from the verb "add," which traces back to Middle English "adden," coming from Old English "addian," meaning "to put on" or "to join." The "on" in the phrase adds a sense of attaching or contributing to something that already exists.