ENQ

ENQ /enkw/ or /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}.