n.a labor contract (now illegal) whereby the employee agrees not to join a trade union
英汉双解大词典
yellow-dog contract
1.
N（名词）
a contract with an employer, now illegal, in which an employee agreed not to join a trade union during his employment 黄犬契约; 旧指企业雇主在雇用劳动者时，以“劳动者不得加入工会”为条件与劳动者签订的契约。现为非法。 [美国英语]