In most Arab countries, the left hand is considered unclean, and it is extremely rude to offer it for a handshake or to wave a greeting. Similarly, it is impolite to pass food or eat with the left hand. If you must know why, let's just say that, historically, people living in deserts didn't have access to toilet paper, so the left hand was used for "hygienic functions," then cleaned by rubbing it in the sand.
Helen Davies, Marjorie Dorfman, Mary Fons, Deborah Hawkins, Martin Hintz, Linnea Lundgren, David Priess, Julia Clark Robinson, Paul Seaburn, Heidi Stevens, and Steve Theunissen
Ever scratch your head when you're frustrated or thinking? HowStuffWorks found out how this tic became the signal for so many social expressions.