On our last trip we bought a collection of artisanal local jams (confitures) and want to provide a translation with the gift, but there’s one causing amusement/consternation in equal measure.
Gratte Cul
Both google translate and Deepl translate this as arse scratcher/butt scratcher. 
The picture on the jar looks like rose hips, but it’s just a drawing. Any thoughts on what this really is, because I don’t believe even even the poorest French family ever made jam from their bottoms. 
Rose Hips!
not sure this is the same recipe, but a neighbour made some of this one year… delicious
The translators call those cynorrhodon (fruit de l’églantier), although the dictionary that I’ve just discovered with google translate says “Réceptacle rouge renfermant les fruits (gratte-cul) du rosier et de l’églantier.”
Thanks Stella. 
Because you can make itching powder out of the seeds, and presumably if you don’t take the seeds out before you make the jam, they may be itchy in the way out.cynorrhodon is the botanical name, fruit or baie d’églantier is the polite name and gratte-cul is the common name.
… and do you?
Haven’t even discussed it for many years, you just scooped the seeds out and shoved them down the back of your victim’s neck, as far as I remember. They are covered in little hairs which I suppose is the itch generator.
In that case, I won’t ask at what age you stopped deploying it
You can, 7 was the cut-off for me, I never actually used it myself, it was something I saw the bigger boys doing, like burning little circles in outdoor benches with a magnifying glass or riding a bicycle without using your hands.
At my next prep school the girls were a bit feeble really, or possibly just more modern.
And of course many French adults with very expensive bike still do it (used to love it too when I was a teenager)
We used to do that as kids, you just bought back a nasty memory of being almost on fire under teeshirts.