[BadbarmrapBef]

That very interesting, Meng,
Why "spiritually" dirty?

One's feet tread on the earth, apart from the things like dirt and crap you could step on(hence, the custom of removing your shoes before entering a household), you also tread on other things which are considered unclean. For example, after you go to a funeral it is said you should wash your bare feet before you enter your house. This is to wash away whatever 'impurities' you may have picked up in a place of death. By 'impurities' one can view it as negative emotions in the air(the depressing atmosphere of a funeral), or restless spirits one might encounter. There's a little superstition/belief that spirits may follow your footsteps/prints back to your home and that by washing your feet. You sever the trail.

Your feet then, comes in direct contact with a lot of bad things on the earth/ground. It is therefor, the most unclean part of your body.

Hmmnnn...how can I explain this...my mother taught me this old saying when I was a kid.

"Your belly button is your link with your mother/the womb and your head(the crown) is your link to heaven."

Your head then, is a very holy part of your being. So putting your feet on somebody's head is seen as disgusting.