The phrase "clean as a whistle" means being completely clean or free from dirt, impurities, or moral wrongdoing. It suggests an immaculate state or a situation that is free from any blemish or guilt. The expression is often used to describe a person, place, or thing that is spotless or in perfect condition.
The origin of the phrase "clean as a whistle" is somewhat unclear, but it is believed to date back to the late 19th century. The word "whistle" refers to the high-pitched sound produced by forcing air through a small opening, such as a whistle instrument. The idea may be that a whistle, when clean, produces a clear sound, symbolizing purity and clarity.