where/how did this originate?
There are a number of theories out there, but the definitive answer has yet to be found.
Some believe the tradition stems from the ancient Druids, who regarded mistletoe with much esteem. They believed it would protect its keeper from evil, provide an antidote to poison and cure illnesses. Branches of the plant were used to bring in the New Year, which may be why we use mistletoe to decorate for the holiday season today.
The first recorded case of someone actually kissing under the mistletoe occurred in 16th century England. However, some say the premise evolved much earlier than that when people believed the plant had powerful effects on fertility and conception.