A man who's wife is f**ked by other men, but it's been adopted by the alt right and is generally thrown around as a derisive term for those that believe in any form of political correctness or the values that underlie it, like equality , that kind of thing. Often just shortened to cucks or cuck.
Given where it's popularity of use has grown from, the alt right basically evolving from a bunch of white supremacist types on reddit, you might question why anyone would want to use the term, or promulgate the politics associated with it.