Oh ^_^ its an informal expression meaning " not a thing" or " nothing at all" So this question here of " "Does mum know? " -"Not a whistle" means that mum doesn't know so if it was said normally ( because I don't think that expression is used very often even though anyone would understand you if you said that )- it would be " Does mum know? " Not at all"

I have never heard this expression and I would never use it.