ACK

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").