George Murchinson and Walter have just had a discussion about Walter's dream. Walter makes snide remarks to George about his white shoes and college prep style. Walter is a bitter black man who feels he has never had a break in life. Of course, Walter has just come home drinking and George is sitting on the couch waiting for his date with Beneatha.
Walter begins taking out his frustrations on George. George maintains a calm and collected attitude. After Walter has fussed and cussed and said what he wanted to say to George, he retires for the evening. After Walter is out of earshot, George says "Good night, Prometheus." Prometheus is a character who suffered much but was being punished for foing wrong:
Prometheus, the god who was punished for having brought fire to mortals, was chained to Mt. Caucasus, where his liver was torn out every day by an eagle but grew back each night. Prometheus' suffering lasted for thousands of years — until Hercules killed the eagle and freed Prometheus.
George is more or less being sarcastic and showing off his knowledge. However, Walter's character of having a fiery personality, wrong behavior and much suffering in his life is parallel the character of Prometheus.
George is pedantic, showing off his knowledge, when he says to Walter (after he is safely half-out the door), "Good night, Prometheus."