The Dallas Cowboys football team has plenty of fans, but also plenty of people who like to root against the team. The “ABC” nickname—“Anybody But the Cowboys” or “Anyone But the Cowboys”—has been cited in print since at least the 1990s.
 
   
Wikipedia: Dallas Cowboys
The Dallas Cowboys are a professional American football franchise that plays in the Eastern Division of the National Football Conference (NFC) of the National Football League (NFL). They are headquartered in Valley Ranch in Irving, Texas, a suburb of Dallas. The team plays its home games at Cowboys Stadium in Arlington, Texas, within the Dallas–Fort Worth metropolitan area, which finished construction in time for the 2009 season. The Cowboys joined the NFL as a 1960 expansion team. The team’s national following might best be represented by its NFL record of consecutive home sell-outs. The Cowboys’ streak of 160 sold-out regular and post-season games began in 1990, and included 79 straight sellouts at their former home, Texas Stadium, and 81 straight sell-outs on the road. The franchise shares the record for most Super Bowl appearances (8) with the Pittsburgh Steelers, corresponding to most NFC championships (8). The Cowboys are the only NFL team to record 20 straight winning seasons (1966–85), in which they only missed the playoffs twice (1974 and 1984), an NFL record that remains unchallenged.

An article from Forbes Magazine, dated September 5, 2012, lists the Cowboys as the highest valued sports franchise in the history of the United States, and second in the world (behind Manchester United of the English Premier League), with an estimated value of approximately $2.1 billion. They are also the wealthiest team in the NFL, generating almost $269 million in annual revenue.
 
13 January 1995, Fort Worth (TX) Star-Telegram, “Anti-Cowboys pick a good bet for Dallas fans,” Metro, pg. 19:
For years I’ve been known around the Metroplex as the “ABC (Anybody But the Cowboys) Man.”
 
15 December 1995, Lexington (KY) Herald Leader, “NFL Favorite Easy as ABC: Anyone But the Cowboys,” pg. C1:
I’m an ABC man—Anyone But the Cowboys.
     
Google Groups: alt.sports.football.pro.dallas-cowboys
Karen King
1/19/96
Hey, ABC Club—The Boys are Ready
(...)
So, come on all of you Cowboy Haters, all of you “Anyone But the Cowboys” club members.  You can’t stand it.  Once again, they’ve risen above the adversity, they shined on the line, and they perserved.
 
21 August 2006, Las Cruces (NM) Sun-News, “Sound Off!”:
This is for the individual who told the Bronco fan that this is Cowboy country. I beg to differ with you. Cowboy country is in Texas. You, my friend, are in New Mexico. A state that has no NFL team and this is an ABC state — Anybody But the Cowboys.
 
Sports Ecards
07.28.2012
I am an ABC fan…
Anybody
But the
Cowboys!!!!!
 
Twitter
Mia Drane-Maury‏
@miadranemaury
ABC. Anybody but the Cowboys.
10:29 PM - 30 Dec 12
 
The Landry Hat
The Renny Rant: Top 10 Reasons Why People HATE Our Dallas Cowboys
May 15th, 2013 at 3:00 pm by Renny Mason
(...)
There is even a common slogan most Cowboys haters’ follow known as the “A.B.C. Method.” This of course stands for “Anybody BUT the Cowboys.” Just as we Cowboys fans grew up soaking up the ‘Boys and all their glory and falling in love, others were trained “You root for our team AND WHOEVER Dallas is playing.” So it would then seem most Dallas haters’ have indeed TWO favorite teams.