The Jargon File (version 4.4.7, 29 Dec 2003):

ACK
 /ak/, interj.

    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
    box?? ?ACK!?

    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 (30 December 2018):

ACK

   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.

   [Jargon File]

   (1997-01-07)