Old-school BBS term. In the days of dialup bulletin board systems, most of them were single-line, which meant that during prime-time 5-11pm, there would be a few dozen users trying to get on all at the same time, where each one would stay on for half an hour to read and reply to that day's messages and possibly playing some BBS Door games.
... and so on.
is the feature of terminal emulator
, busy signal
, lather, rinse, repeat
until it gets the line... and hopefully being there to catch it.