Definitions of nak  

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. “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!”