Clean as a whistle means perfectly clean, or completely free of blame. The record was clean as a whistle.
The moral sense is the commoner and is used of records and investigations. A whistle only sounds clear when it is empty, which is where the comparison comes from.
1phraseclean as a whistlePerfectly clean, or completely free of blame.
“The audit came back clean as a whistle.”
clean as a whistle is a phrase.
Practised in Idioms