Argo who? Argo f**k yourself!

Calloway, who was always telling jokes, had a favorite that had made its way into our offices. It was a knock-knock joke involving a drunk at a bar. "Knock-knock," Calloway would say. "Who's there?" the person would respond. "Argo," he'd say. "Argo who?" the person would dutifully answer. "Argo f**k yourself!" he's excalim, drawling out the words as if he's justed finished a bottle of bourbon. It wasn't long before the punch line had become a battle cry of sorts when the workload was heavy and we had a lot on our minds. Whenever that happened, Calloway would cut the tension by shouting out "Argo!" at which point everyone would respond in kind.

I read through this paragraph many times, but I do get the laughing point ot the humor of Calloway joke. What's the point of humor in Calloway's joke?