The American Broadcasting Company (ABC) began in 1943 and is headquartered on Manhattan’s Upper West Side (near Lincoln Center). ABC is often called the “alphabet network” because its letters are the first three of the English alphabet. The ABC nickname of “alphabet network” has been cited in print since at least 1971.
 
Other ABC nicknames include “All Bullshit Channel,” “Almost Broadcasting Company,” “Already Been Canceled,” “Always Broadcasting Crap”  and “Awful Broadcasting Company.”
 
The “Alphabet Network” is sometimes said to be a part of an “Alphabet Mafia” of media companies.
   
 
Wikipedia: American Broadcasting Company
The American Broadcasting Company (ABC) is an American commercial broadcasting television network. Created in 1943 from the former NBC Blue radio network, ABC is owned by The Walt Disney Company as of 1996 and is part of Disney-ABC Television Group, formerly ABC-TV. Its first broadcast on television was in 1948. It is the largest broadcaster in the world by revenues. As one of the Big Three television networks, its programming has contributed to American popular culture.
 
Corporate headquarters is in the Upper West Side of Manhattan in New York City, and the company’s news operations are also centered in Manhattan. Entertainment programming offices are in Burbank, California adjacent to the Walt Disney Studios and the corporate headquarters of The Walt Disney Company.
 
The formal name of the operation is American Broadcasting Companies, Inc., and that name appears on copyright notices for its in-house network productions and on all official documents of the company, including paychecks and contracts. A separate entity named ABC Inc., formerly Capital Cities/ABC Inc., is that firm’s direct parent company, and that company is owned in turn by Disney. The network is sometimes referred to as the “Alphabet Network”, due to the letters “ABC” being the first three letters of the Roman-Latin alphabet, in order.
 
19 October 1971, The Plain Dealer (Cleveland, OH), “ABC-TV Comes On Strongly in Ratings” by William Hickey, pg. 10-A, col. 5:
As it is, ABC-TV is dominating two evenings a week and taking its share o a third. Perhaps the toughest lineup in all of prime time is the Alphabet Network’s Tuesday evening triumvirate of “Mod Squad,” “Movie of the Week” and “Marcy Welby, M.D.”
 
17 November 1971, The Plain Dealer (Cleveland, OH), “ABC, CBS announce new shows” by William Hickey, pg. 7-F, col. 4:
The alphabet network (ABC) is canceling four shows ...
 
19 October 1983, Boston (MA) Globe, “Olympian coverage and rates for the games” by Robert A. McLean, Economy, pg. 1:
ABC-TV Vice President John Martin was describing the 1984 summer Olympic Games in Los Angeles, which, with the winter games at Sarajevo, Yugoslavia, should be a bonanza for the alphabet network.
   
Google Books
The Incredible Internet Guide to Howard Stern
By James R. Flowers
Tempe, AZ: Facts on Demand Press
2000
Pg. 59:
The alphabet network has been known for using offbeat marketing initiatives to promote its lineups, but this takes the cake. In order to hype the return of “Norm”, ABC is installing several hundred high-tech urinal billboards with pictures of Our Hero in men’s restrooms throughout New York City and Los Angeles.
 
Google Books
Anchoring America:
The Changing Face of Network News
By Jeff Alan and James Martin Lane
Chicago, IL: Bonus Books
2003
Pg. 112:
In 1981, Roone Arledge asked Brinkley to join ABC as a Washington insider, and he took to the air on the alphabet network with This Week with David Brinkley.
   
Google Books
Characters of the Information and Communication Industry
By Richard F. Bellaver
Bloomington, IN: Authorhouse
2011
Pg. 207:
The TV movie was also invented at ABC-TV, and in time the “alphabet” network received top ratings for airing Brian’s Song, The Thorn Birds, and The Winds of War.