Negative interest rates mean that depositors are charged for holding money with commercial banks, acting as a fee on deposited funds rather than earning interest. This is a matter of definitional economic terminology and general knowledge, requiring no specific citation.
The claim provides the standard economic definition of negative interest rate policy (NIRP), describing how it applies to depositors. Because this is a definitional statement of financial terminology rather than a contestable empirical finding, it is classified as common knowledge.