Etymologically, cuckhold used to refer to someone being cheated on without their knowledge and consent (and still has that meaning in the dictionary, though its usage is rather archaic now). Using it to refer to the fetish is somewhat more recent. Historically, it has been used as a derogatory term or insult in certain cultures and time periods. So their usage of it isn't exactly unprecedented, though it is odd that it's been latched on to as a generic insult. https://en.wikipedia.org/wiki/Cuckold