Like the engine, the straight 6 is six men in a line involved in anal intercourse. Unlike the rods and cylinders going up and down in an engine the straight 6 men move their hips back and forth in unison until climax.
Disclaimer: there is NOTHING “straight” about this except the line of men.
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.