From the same wiki article: Beginning in the early twentieth century, the terms "left" and "right" came to be associated with specific political ideologies and

From the same wiki article: Beginning in the early twentieth century, the terms "left" and "right" came to be associated with specific political ideologies and were used to describe citizens' political beliefs, gradually replacing the terms "reds" and "the reaction". The words Left and Right were at first used by their opponents as slurs.