In the case in 1908, the supreme court declared that the sherman anti-trust act could be used against trade unions as well as business monopolies.

The case that the question is asking about is Loewe v. Lawlor.

Loewe stands for D. E. Loewe & Company, a company producing hats and Lawlor represents the United Hatters of America, a union of hat makers, that encouraged the consumers to boycott Loewe, who was not a part of the union. Loewe sued Lawlor, and won.