A message that those of us old people who used to get on BBSes back when they were not a message board on the internet but another person's computer that we called up with our modem and a terminal program and entered a command like "ATDT555-1212" which caused the modem to dial and connect.

It meant that you were disconnected. If you intended to log off then it was ok. But if someone picked up the phone, or for who known what reason, sometimes you'd get it in the middle of doing something. Maybe even during an online game where you were in fierce competition with someone and because there was only one telephone line for the BBS whoever got in first had the advantage and when you got cut off often someone else would get on instead and you may not be able to get back on, or if so, until your rivals were done leaving you in a worse position.

Also a fun thing to put in your terminal emulator's macro set to be quickly autotyped into chat systems of yore, or in a private convo with someone. It only worked once or twice, though, much like the boy who cried wolf. But for those few times it was amusing to think that elsewhere in your area code, an undetermined number of people were jumping up to check their acoustic couplers or modem blinkums.