It means a man with a sexually unfaithful wife. Even the concept has sexist, homophobic, and misogynous implications that I may explore later under a “women as property” theme, but I suspect the reason “cuck” is an attractive term to conservatives of a politically incorrect mindset is the alliteration and assonance with a certain, more vulgar, derogatory term, and it rhymes with another.
Origins (deeply historic) and currency in contemporary conversation aside (I am not an English Major) the secondary level of insult is that the target is not “manly” enough to singularly fascinate his mate (again females are considered incapable of independent action and are instead masturbatory mannequins to be manipulated).
And now we have this Neo-Nazi (and we will never on these pages dignify them in our commentary as ‘alt-right’ though we may quote those who do) anti-Semetic spin. Where the cuck does that come from?
I hesitate to call it a rant because Cenk Uygur is a very reasonable gentleman who hardly ever raises his voice above a modulated baritone, but for your consideration in lieu-