Why Six Feet Under?
Well, in 1665 the 'Black Death' (bubonic plague) killed 65,000 Londoners and so the Lord Mayor of olde Londinium created new 'rules' for disposing of the dead. Shallow burials were now 'out' and 'in' came these new fangled graves that were to be at least six feet deep. Since then this depth has been the standard.
Also round about this time fleas and body lice were rampant and so most people shaved their body hair. The upper classes were not happy with their new bald pubic look and so the merkin was invented, a triangle of 'real' fur that they stuck 'down there' to hide their embarrassment. As if a stick on fanny wig isn't embarrassing enough. 100 years later the word was adopted by soldiers to describe the mop that they used to clean out their canons.