Monger is a slang term for an individual who regularly employs the services of sex workers. It is a shortening of the term whoremonger.
Like the dictionary term, the slang term monger refers to a person who is involved with something in an unbecoming or disreputable way. For example, just as a gossipmonger is seen as a troublesome gossip, a whoremonger is criticized for their frequent dalliances with prostitutes.