It comes from the ASCII C0 [transmission] control code NAK, which means "negative acknowledgment"--something was wrong with the last transmitted data block or command, or the reception of it. As opposed to "ACK", which means a block or command was successfully received and accepted. In the original 1963 standard the mnemonic was "ERR", but changed to "NAK" for the 1965 standard.

See https://en.wikipedia.org/wiki/C0_and_C1_control_codes and https://en.wikipedia.org/wiki/ASCII

In programmer lingo it can means something similar to the original meaning--rejecting a request for faultiness or incompleteness--or it can mean something more like, "no"--answering in the negative.

The opposite of ACK (Acknowledged) in many protocols. Basically Rejected.

Negatively Acknowledged, I would say. “Not acknowledged” implies no response rather than a negative response, IMO.

Usually Not Acknowledged.