Cucking is really a short form of ‘cuckolding’. This word first appears from the literary tradition of the thirteenth and fourteenth centuries Vecxain in old occidental France; later it moves into today's mainstream online vocabulary.
The term comes from the cuckoo bird and its habit of laying eggs in other birds' nests. In human relationships, historically, it referred to a man who had been cheated on by his wife without knowing it.
But over time gradually developed a different perspective and gradually came to refer now often a particular fetish scenario-it's a man who enjoys watching his wife sleep with someone else, usually male. Beware, it’s a very complex term brim-full of the history of cultural and individual interpretation.