From The Jargon File (version 4.4.7, 29 Dec 2003):
tube
1. n. A CRT terminal. Never used in the mainstream sense of TV; real
hackers don't watch TV, except for Looney Toons, Rocky & Bullwinkle, Trek
Classic, the Simpsons, Babylon 5, and the occasional cheesy old
swashbuckler movie.
2. [IBM] To send a copy of something to someone else's terminal. “Tube me
that note?”