CBS (the initials of the original name of the Columbia Broadcasting System) is an American broadcast television and radio network. “Constant Bull Shit” is an unflattering nickname based on the three letters.
   
“CBS (Constant Bull Shit)” was posted on the newsgroup alt.books.stephen-king on July 20, 1996. ” CBS ( Constant B.S Network)” was posted on the newsgroup alt.republican on September 20, 2004.
     
Other CBS nicknames include “Complete Bull Shit” and “See B. S.”
     
     
Wikipedia: CBS
CBS (an initialism of the network’s former name, the Columbia Broadcasting System) is an American English language commercial broadcast television and radio network that is a flagship property of CBS Corporation. The company is headquartered at the CBS Building in New York City with major production facilities and operations in New York City (at the CBS Broadcast Center) and Los Angeles (at CBS Television City and the CBS Studio Center).
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The network has its origins in United Independent Broadcasters Inc., a collection of 16 radio stations that was purchased by Paley in 1928 and renamed the Columbia Broadcasting System.
       
Google Groups: alt.books.stephen-king
OT Gary Cole (Was: Re: American Gothic - I see Flagg!) 
Suzanne Winn
7/20/96
(...)
Yes, CBS (Constant Bull Shit) originated the show and has subsequently cancelled it after virutally no promotion and a switch in time slots.
 
Google Groups: alt.rush-limbaugh
CBS=Clinton Bull Shit
BJ
8/24/99
(...)
CBS ... Constant Bull Shit
     
Google Groups: rec.autos.sport.nascar
Race Replay?
jstark
7/4/00
Oh… and I thought CBS stood for Constant Bullshit Network…. guess I was
wrong…. or was I?
   
Google Groups: alt.republican
Acts of treason by Dan Rather
Geoman
9/20/04
We need a Senate hearing to investigate if crimes of treason have been committed by the DNC, the Kerry Camp, Dan Rather and his CBS ( Constant B.S Network)
     
Twitter 
Paige S.
@11Phoenix
Constant Bull Shit.. that’s all. RT @dcconfidential: CBS President Warns of More Daytime Changes, Cites ATWT http://bit.ly/BmPIY
8:47 PM - 3 Aug 2009
 
Twitter
LK Sugarman
@lksugarman
CNN stands for Constant Negative News. CBS stands for Constant “B.S.” NBC stands for Negative B.S. and ABC stands for Absolute B.S. Love it!
8:46 PM - 28 Sep 2009
     
Twitter
Laura Wilson
@tciddaisc
Replying to @bryn_elizabeth
@bryn_elizabeth Yeah? Well I came up with the new name first. CBS: Constant Bull Shit. Or Constant Bullshit Station, whichever you prefer.
11:04 PM - 15 Jun 2010
 
Twitter
❌ toddgriz ⭐️ ⭐️ ⭐️
@ToddGriz
CBS=CONSTANT B.S.=CBS
7:40 PM - 13 Apr 2019
 
Twitter
alvinjn
@alvinjn
Replying to @RealSaavedra @GrrrGraphics
CBS = Constant Bull Shit!
11:08 AM - 20 Apr 2019