If something is As Clean As A Whistle it's bright, shiny and spotless. It can also mean untarnished in the sense of escaping from somethings "as clean as a whistle". There are a few suggestions as to the phrase's origin. One is that it lies with the bright shiny locomotive with its polished brass whistle, whilst another is that it refers to a freshly carved wooden whistle or to the sound a shining sword makes as it swishes through the air.