The Jargon File (version 4.4.7, 29 Dec 2003):

NAK
 /nak/, interj.

    [from the ASCII mnemonic for 0010101]

    1. On-line joke answer to ACK?: ?I'm not here.?

    2. On-line answer to a request for chat: ?I'm not available.?

    3. Used to politely interrupt someone to tell them you don't understand
    their point or that they have suddenly stopped making sense. See ACK,
    sense 3.

    3. ?And then, after we recode the project in COBOL....? ?Nak, Nak, Nak! I
    thought I heard you say COBOL!?

    4. A negative answer. ?OK if I boot the server?? ?NAK!?


The Free On-line Dictionary of Computing (30 December 2018):

NAK

   Negative Acknowledgement