Great history lesson here by a blogger named Suldog about the origins of the "Beat LA" chant:
For Celtics fans of that time period, the 76’ers were the team to beat. Crucial to the understanding of this story, however, is the fact that the Celtics and 76’ers respected each other wholly. So did the fans of both. They were enemies, but they were enemies who had earned their due.
In 1980, Philadelphia beat Boston in the semi-finals, earning a trip to meet the Lakers for the championship. In 1981, Boston beat Philadelphia, coming back from a three-games-to-one deficit. In 1982, they met once again in the semi-finals, and here is where the tale becomes more than just your usual sports story.
As always between these two teams, the 1982 series was an all-out tong war. There was little to separate the two squads. The Celtics had Larry Bird, Kevin McHale, Robert Parrish, Cedric Maxwell, Danny Ainge and Tiny Archibald. The 76’ers had Julius Erving, Bobby Jones, Maurice Cheeks, Caldwell Jones, Darryl Dawkins and Andrew Toney. And, again, it came down to a seventh game, this time being played at the old Boston Garden.
The Garden was packed to the rafters, hot and muggy, as it usually was during the later rounds of the playoffs. Both teams battled hard, as they always did. The game went back-and-forth, one team gaining momentum and then the other. In the fourth quarter, as the minutes ticked down and it became obvious that the 76’ers - not the Celtics - were going to The Finals, a wondrous thing occurred. It started softly, but grew to a deafening roar.
The Boston Garden crowd started chanting, with no prompting from a giant scoreboard, or from cheerleaders, or due to any sort of pre-packaged canned marketing.
What they started chanting was "Beat L. A.! Beat L. A.! Beat L. A.! Beat L. A.!"
In the midst of a heartbreaking defeat, they were cheering on their most hated rivals.
They were, at that moment, the classiest fans in all of sport.