So the boys and I were watching "Blackadder Goes Forth," which is of course set during WWI, and which you should absolutely see if you've not yet had the opportunity.
Partial transcript from the episode "Goodbyeeee!":
George: All right, so, what do we do now?
Baldrick: Can I do my war poem?
Edmund: How hurt would you be if I gave the honest answer, which is "No, I'd rather French-kiss a skunk"?
Baldrick: So would I, sir!
At which Rhys pipes up: "What's a French kiss?'
I am preparing an answer, when Fisher busts in for me, with an air of authority which is impossible to reproduce here:
"It's when you kiss someone on both cheeks."
Thank goodness for children who enjoy answering other children's questions.
(Wow. You can actually watch the whole episode in question here.)