Not worth the paper it’s written/printed on
Definition 1 of 1
Pro Tip 1/3Lack of Credibility
Used to express that a document or statement is not trustworthy or reliable. 
Their guarantee turned out to be not worth the paper it's printed on.
Pro Tip 2/3Hyperbolic Expression
It's an exaggerated way of saying something is completely useless or ineffective. 
His vows of loyalty were not worth the paper they were written on.
Pro Tip 3/3Legal Disputes
Often invoked when formal agreements or contracts fail to hold up in practice or law. 
The non-compete clause is not worth the paper it's printed on.