In the jewish tradition rabbis may not touch women other than their wife. Thus when greeting a new woman they simulate hug without actually touching.
This is adapted for situations between friends when u are at a distance from one another but still wish to give the love and support of a big fat hug :-)
*one sees ones friend across the room looking a little down in the mouth and po-faced* you catch their eye say 'rabbi hug' and the rabbi hug ensues. Then warm fuzzy feelings come floating over you.