When reading This post on ’tis the season, a phrase stood out. Not for it’s factual nature as you will see in this quote:

mistletoe (Appox, 13th century Old English for “Like the toe of the missile” in that it looked like the arse-end of a warhead)

It (of course, you should know by now) lead me to curiosity to find out about mistletoe, not just the name but why are we supposed to kiss under it?
Here is what I found:

Firstly mistletoe got it’s name from the 2nd century Anglo-Saxons. Originally called ‘Misteltan’, which is the word ‘tan’ meaning ‘twig’ forcably abutted to the word ‘mistel’, meaning ‘dung’. See they thought that the ‘misteltan’ was caused by birdshit on trees. Of course we know now that it’s the mistletoe seed in the birdshit that causes the growth.
Thats the name sorted out – twig-dung, or dung-on-a-twig, which of course adds even more mystery to the reasoning behind kissing under it.

As with all things from that era, furtility comes into everything somewhere. I’ll come onto that in a moment though, first I’ll explain the Viking reason behind the tradition:
Some guy called Balder had a dream that he was going to die, dreams must’ve been considered a pretty powerful thing in those days because this completely freaked his mum (Frigga) out. So in order to protect him she went to each of the elements – earth, air, fire, water, and all the plants and animals – and asked them not to kill her son.
Apparently a lot of people took the piss out of Balder for running to his mum and threw stuff at him. Not entirely sure where that comes into things but it seems that everyone thought he was immune to harm. So thats good!
To cut a story short (I’m getting bored of typing) mistletoe’s parasitic nature offered a loophole in the protection so Balders arch enemy Loki used it to kill him. The elements tried to resurect him but after three days only white mistletoe berrys would do the trick. So in Viking lore, mistletoe can bring people back from the dead, and Frigga (the mum) kissed everyone who walked under the mistletoe to show her gratitude.
For the Druids of Britain the mistletoe was simply amazing, if they had sliced bread it would undoubtably have been the best thing since. You see they thought mistletoe could perform miracles, anything from fertility (of course, they were druids after all) to healing desease, prtecting against witchcraft and warding off evil spirits. They had a whole ceremony involving picking mistletoe and killing while bulls at solstace time.

Mistletoe would be completely great and amazing and fantastic if it could do any of these things, but it can’t.

Still it’s a good excuse to try and get a kiss at Christmas time. Before I leave you reeling with all this lovely knowledge, I’d best explain proper mistletoe etiquette: After the kiss, the man should pick a berry from the sprig. Where there are no more berrys, there is to be no more kissing!
It is also believed that an unmarried woman NOT kissing under the mistletoe will remain single for another year. A rule probably thought up by men. I do like the fact that it says nothing about marrying the guy you kiss – even more evidence that it’s probably ‘a guy rule’.

Sources:
How stuff works
‘Gardenline’