Wordorigins.org

A French kiss is an open-mouthed kiss with tongue. It’s first recorded as verb. From Linder’s Letters of 1918:

A sort of liaison between tongues (not to be confused with French kissing).

But why French? The French have been associated with sexual practices dating back to the 18th century. From Henry Fielding’s 1749 Tom Jones:

I am so far from desiring to exhibit such Pictures to the Public, that I would wish to draw a Curtain over those...n certain French novels.

In this case, Fielding was writing about risqué novels that were literally French. By the mid-19th century, the figurative sense was well established. From Robert Browning’s 1842 Bells and Pomegranates:

My scrofulous French novel On grey paper with blunt type!

(Source: Oxford English Dictionary, 2nd Edition; Historical Dictionary of American Slang)