I found this on the net for you. - The prejudice that anything French is wicked, sexual, and decadent has let Frenchmen in for more than their fair share of abuse in English. Many such expressions date back to 1730-1820, the height of Anglo-French enmity, but some are current and others go back even further." From the "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Fact on File, New York, 1997).
: : : "French Kiss" is an obsolete term for what the hyperpatriotic Americans now call a "Freedom Kiss":>
: : FRENCH KISS - "a passionate open-mouth kiss with tongues caressing. This term has probably been in use since World War I, but became extremely popular with teenagers during the late 1930s." From "I Hear America Talking" by Stuart Berg Flexner (Von Nostrand Reinhold Co., New York, 1976).