A response that neither confirms nor denies the existence or nonexistence.
My source is
DOD regulation 5400.7 paragraphs C18.104.22.168.5 and C22.214.171.124.1.3.1 These paragraphs have to do with the freedom of information act (FOIA). It is a method of denying information considered exempt by the act.
The first time was used was a response from the CIA to a FOIA request. It was dealing with the Glomar Explorer, a ship supposedly built to recover a sunken russian sub. That is how it got its name.
The accused responded with a glomar response. His exact words were "I can neither confirm nor deny my presence at that donut shop during they day and time stated."