Well thE definition is the husband of an adulterous wife. It is derived from the habit of the cuckoo bird, whose females would lay eggs in other birds nests, and then those birds would raise the offspring. So really most ducks are unknowing.
As a fetish, it’s dudes that like to watch their chicks get banged out by another dude. Some say it’s a form of humiliation and masochism that gets the guy off. Others say it has a gay element, as when the husband finishes the guy off or his “DNA” after the fact.
On Internet boards, it’s the new “
”.