6 o'clockhot is where the temperature is so high, you can't do anything until 6:00pm in the evening - you just want to lay low and as close to air conditioning as possible!
Cedric The Entertainer often uses this expression to describe the Southern states of the USA.
Referring to the appearance of a woman who has absolutely nothing to offer in the chest. A woman who is Straight up and down like six o clock looks exactly like what the term implies---there are no boobs to break up the straight line appearance when you look at her from top to bottom. Most prominent when viewed from the side.