The opposite of ACK. NAK is a slang term that means that you disagree, or do not acknowledge something. This also refers to the 21st ASCII character.
An "negative" acknowledgement (disapproval) from an authorized point of contact for a domain name record or contact record, that a requested modification should not be completed. When a modification request is received from a source that is not an authorized contact, the request will be rejected and a message will be sent to the listed contacts asking them to ACK (approve) or NAK (disapprove) the modification. [see also: ACK (Acknowledge), Modification, Contact/Agent
See: Negative Acknowledgment
Negative Acknowledgment. NAK is a control character in ASCII that means a packet arrived with the check digits in error. It is sent from the computer receiving the packets to the sender, implying that the packet should be retransmitted so that all bits will arrive intact next time.
Negative Acknowledgment. See ACK
A control code indicating that a character or block of data was not properly received. The name stands for negative acknowledgement. See ACK. ... more
(knack) A response that something was not received properly. Short for Not AcKnowledged. Ironically, the mere fact that there's a NAK implies that the packet or command was received. It could be that there was some sort of an error (possibly checksum related) or that the requested command was not supported.
negative acknowledgment. Response sent from a receiving device to a sending device indicating that the information received contained errors. Compare to acknowledgment.