"Cuckold" has taken on a broad meaning lately. It had originally meant a man who is cheated on by his wife, possibly for his own fetishistic gratification. When applied in a political context it essentially denotes one whose political goals conflict with Western ideals of individualism, private ownership, nationalism, or tradition. As when you'd let another man impregnate your wife, so to are mass immigration and similar movements that are deleterious to Caucasian identity a permission to let others destroy that identity. A liberal, who is usually more concerned with West-detrimental political correctness, is more likely to espouse the "cuck" mindset that the rising alt-right denounces.