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

ENQ
 /enkw/, /enk/

    [from the ASCII mnemonic ENQuire for 0000101] An on-line convention for
    querying someone's availability. After opening a talk mode connection to
    someone apparently in heavy hack mode, one might type SYN SYN ENQ? (the
    SYNs representing notional synchronization bytes), and expect a return of 
    ACK or NAK depending on whether or not the person felt interruptible.
    Compare ping, finger, and the usage of FOO? listed under talk mode.


The Free On-line Dictionary of Computing (30 December 2018):

ENQ

   1.  /enkw/ or /enk/ ENQuire.  The mnemonic for
   ASCII character 5.

   2.  An on-line convention for querying someone's
   availability.  After opening a chat connection to someone
   apparently in heavy hack mode, one might type "SYN SYN ENQ?"
   (the SYNs representing notional synchronisation bytes), and
   expect a return of ACK or NAK depending on whether or not
   the person felt interruptible.

   Compare ping, finger.

   [Jargon File]

   (1998-01-18)