What is cuckold fetish?

Cuckold is a derogatory term for a man whose wife is or has been unfaithful.

so the fetish would be...

Cuckold Fetish is a form of BDSM play where the fetishist gets sexually excited by the idea of their partner having sex with someone else. It is distinguished from open relationships or swinging by being one-sided (the fetishist is not free to see others) and by the dimension of humiliation or psychological pain. Fetish is the eroticization of fear, and in the case of a Cuckold Fetish, the fear is that you will be betrayed, rejected, or otherwise dismissed by your partner in favor of another.

Fetish cuckolding takes its name from a traditional epithet for a man whose wife has secretly had another sexual partner. In the fetish, however, the the cuckold knows (technically making him a wittol rather than a cuckold) although the conceit of non-consensual or compulsion is often important among fetishists.

The "cuckold" is the partner who is not having the extracurricular sex. The Word's traditional usage applied only to married heterosexual men, but in the fetish scene can mean a partner of any sex in any form of committed pair-bond relationship.