Source : Free On-Line Dictionary of Computing

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)