ACK: /ak/ interj. 1. [from the ASCII mnemonic for 0000110] Acknowledge.

:ACK: /ak/ interj. 1. [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".

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}
(sense 2), i.e., "I'm not here").
-- The AI Hackers Dictionary