The Jargon File (version 4.4.7, 29 Dec 2003):
1. [common; from the ASCII mnemonic for 0000110] Acknowledge. Used to
register one's presence (compare mainstream Yo!). An appropriate response
to ping or ENQ.
2. [from the comic strip Bloom County] An exclamation of surprised disgust,
esp. in ?Ack pffft!? Semi-humorous. Generally this sense is not spelled in
caps (ACK) and is distinguished by a following exclamation point.
3. Used to politely interrupt someone to tell them you understand their
point (see NAK). Thus, for example, you might cut off an overly long
explanation with ?Ack. Ack. Ack. I get it now?.
4. An affirmative. ?Think we ought to ditch that damn NT server for a Linux
There is also a usage ?ACK?? (from sense 1) meaning ?Are you there??, often
used in email when earlier mail has produced no reply, or during a lull in
talk mode to see if the person has gone away (the standard humorous
response is of course NAK, i.e., ?I'm not here?).
The Free On-line Dictionary of Computing (18 March 2015):
1. /ak/ The mnemonic for the ACKnowledge
character, ASCII code 6.
2. A message transmitted to indicate that
some data has been received correctly. Typically, if the
sender does not receive the ACK message after some
predetermined time, or receives a NAK, the original data
will be sent again.