Middle class SWPLs that vote republican and constantly compromise their positions in order to appear not racist/anti gay/mean. Most users of /r/Conservative fit the bill perfectly. The conservatives who think Iran poses an existential threat and worship Israel. The conservative who ignores social affairs because "it doesn't hurt anyone" or "muh consenting adults".

http://www.radixjournal.com/blog/2015/7/16/cuckservative-a-definition

The term came out of one of many reactosphere blogs and sites and was popularized on /pol/, where "cuck" had been a catch-all term for the average pathetic leftist.