The error message given by a modem when the telephone line carrier signal is lost. In the days of BBSs this was a frequent occurrence, often at the very moment someone was typing a sentence. Loss of signal would cause a string of garbage characters to be displayed to the recipient.
In later years this gave rise to an unending
joke, depicting someone getting forcefully interrupted or otherwise cut off the
internet, out of spite, in the middle of a particularly
boring thought, irritating tirade, or some Famous Last Words.