E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Kissing under the Mistletoe.
Balder, the Apollo of Scandinavian mythology, was killed by a mistletoe arrow given to the blind Höder, by Loki, the god of mischief and potentate of our earth. Balder was restored to life, but the mistletoe was placed in future under the care of Friga, and was never again to be an instrument of evil till it touched the earth, the empire of Loki. It is always suspended from ceilings, and when persons of opposite sexes pass under it, they give each other the kiss of peace and love in the full assurance that the epiphyte is no longer an instrument of mischief
A correspondent in Notes and Queries suggests that the Romans dedicated the holly to Saturn, whose festival was in December, and that the early Christians decked their houses with the Saturnian emblems to deceive the Romans and escape persecution.