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

negative acknowledgement

   1.  (NAK) The mnemonic for ASCII character 21.

   Sometimes used as the response to receipt of a corrupted
   packet of information.

   Opposite of acknowledgement.

   2.  (NAK) Any message transmitted to indicate
   that some data has been received incorrectly, for example it
   may have a checksum or message length error.  A NAK message
   allows the sender to distinguish a message which has been
   received in a corrupted state from one which is not received
   at all.

   An alternative is to use only ACK messages, in which case
   the non-receipt of an ACK after a certain time is counted as a
   NAK but gives no information about the integrity of the
   communications channel.

   See also ACK.

   (1997-01-07)