The phrase "Where on earth did you get that?" is an idiomatic expression used to convey surprise, shock, or incredulity about the origin of something. It emphasizes the question "where" by adding "on earth," which intensifies the sense of wonder or disbelief about how or where something was acquired. This phrase is used in English to express that the item or situation is surprising or uncommon, often implying it is difficult to imagine where it could have come from or been obtained. The addition of "on earth" adds emphasis and makes the question sound more intense or curious than just "where" alone would.

In summary, the phrase is a strong expression of surprise about the location or source of something.