I'll defer to Richard Spencer, president of the white nationalist National Policy Institute.
"#Cuckservative” is a full-scale revolt, by Identitarians and what I’ve called the 'alt Right,' against the Republican Party and conservative movement," Spencer explained in an e-mail. "The 'cuck' slur is vulgar, yes, but then piercingly accurate. It is the cuckold who, whether knowingly or unknowingly, loses control of his future. This is an apt psychological portrait of white 'conservatives,' whose only identity is comprised of vague, abstract 'values,' and who are participating in the displacement of European Americans — their own children.
(Here's more on "Identitarianism." "Alt conservatism" is hard to pin down but easy to define: It rejects modernism and libertarianism. It's generally more isolationist than mainline conservatism, and it's certainly much more cool to multiculturalism. In European multiparty politics, this kind of conservatism usually forms the basis for a splinter party. In the United States, where third parties are largely irrelevant, alt conservatism has sought out occasional champions within the GOP. In 2007, Spencer himself was a founder of the Robert F. Taft Club, which hosted speeches and debates by politicians like then-Rep. Ron Paul (R-Tex.). In 2015, it's Donald Trump and the "Make America Great Again!" message that appeals most strongly to alt-cons.)
Is it just about Donald Trump?
According to Spencer, "Trump is a major part of the 'cuckservative' phenomenon — but not because he himself is an Identitarian or traditionalist. His campaign is, in many ways, a backward-looking movement: 'Let’s make America great again!' Why Trump is attractive to Identitarians and the alt Right is: a) he is a tougher, superior man than 'conservatives' (which isn’t saying much), and b) he seems to grasp the demographic displacement of European-Americans on a visceral level. We see some hope there."
Where'd this come from, anyway?
I gave the game away by referring to the National Policy Institute. "Cuckservatism" is about not just politics, but national -- and racial -- identity. BuzzFeed's Joe Bernstein and the New Republic's Jeet Heer have traced the origins of the term to the white supremacist blog The Right Stuff and the right-wing board My Posting Career.
Heer, who delved further than seems healthy into how the Internet created this term, pointed out that the shortened prefix 'cuck' is both "a genre where husbands, often white, watch their wives have sex with other men, often African-American" and "a much-deployed sneer on 4chan the imageboard website."
And Bernstein points out that the first contextual use of the term came from @Drunknsage, who had been a supporter of the #Gamergate crusade against so-called "social justice warriors of the left."
Who are the "cuckservatives?"
You might be one! The hashtag's targets are conservatives who seem to have made peace with elements alien to traditional white Americanism. That could mean the transgender movement; it could mean non-white immigrants. Certainly, criticizing Trump's visit to the border, saying he will alienate certain voters, is a trial run for cuckservative status.
"Just look at them!" said Spencer. "Glenn Beck, Erik [sic] Erickson, Mike Huckabee. They’re mediocrities, or sub-mediocrities. They’re grinning, obese doofuses. No person with a deep soul — no person who wants to take part in a moment that’s idealistic, that’s going to change the world — would want to be a part of 'conservatism.' In a way, the current 'cucks' are the residue of the Bush era. They were the 'conservative' and 'Religious Right' allies of the neoconservatives. They’re still around, for no apparent reason."
What's the opposite of a "cuckservative," and how many of those people are there?
There's no catch-all term, and the answer depends on how you limit results.
If it's just the people using the new term, then it's a limited number of activists online. The white nationalism represented by Spencer has struggled to find footing. Youth for Western Civilization, a student group that attempted to bring millennials on campus into the "traditionalist" cause, burned brightly for a few years, then went inactive.
If you're asking how many people might agree with the underlying argument -- that the conservative movement has accommodated the cultural left for too long -- the answer might be millions. As many as 45 percent of self-identified "conservative Republicans" oppose any legal status for undocumented immigrants -- i.e., they oppose the establishment Republican position, as represented by Jeb Bush and the U.S. Chamber of Commerce.
Obviously, most of the people on the "kick 'em out" side of the ledger would hardly consider that an expression of white nationalism. "Cuckservative" is a frame that might be bigger than its founders intended.