very ...; nothing more ... than this; as ... as it could possibly be
An emphatic expression attached to adjectives or nouns to express an extreme degree, often with a negative or exclamatory tone. Used in casual speech.
His room is unbelievably dirty.
I can't tell you how frustrated I was when I failed the exam.
Derived from the quotative と plus 言ったら (conditional of 言う) and the negative ない, literally 'if one were to say, there is no ...', used idiomatically to express an extreme degree.