The final action of a computer-especially a mainframe-before it is shut down or "brought down" for good. The term comes from the scene in 2001: A Space Odyssey when HAL sings the song "Daisy" as it is being shut down.
This phrase has morphed
and also refers to the end of an event, or project, or an era, such as
"It's time to sing daisy and get ready for a Happy New Year!"