And here's this interesting comment from the Wikipedia article: "Soldiers of the Imperial Grande Armée incorporated the term sans-culotte into their slang, but rather than having a political definition, the new usage was used to mean 'the Scots', doubtless referring to the Scottish custom of wearing kilts (without underwear) rather than any sort of trousers or breeches."
See my edited comment, now with the Wikipedia link. One question I still have is about the final "s". Do the French use "sans-culotte" as a singular when referring to one of these soldiers (un sans-culotte)? Or do they say "un sans-culottes"? And I find I have another question: is culottes related to the French word for "arse": le cul?
According to the Oxford American Dictionary, a sans-culotte (as they give the headword) was "a lower-class Parisian republican in the French Revolution", and came to mean "an extreme republican or revolutionary" (understanding "republican" as one who supports a republic as opposed to a monarchical form of government). And this word, in its political sense, yielded the word sans-culottism. My guess is that these were probably derogatory terms when applied to French revolutionaries.
Also, according to the Wikipedia article, these soldiers did not wear the more fashionable culottes (knee-breeches) but full-length trousers (Wikipedia has a couple of pictures from the era). They weren't naked from the waist down.
"SANS-culotte, a man without breeches." It sounds funny, but it's probably an official military term because of how infrequently soldiers were supplied with clothing. They literally wore their clothes to rags, and sometimes had to walk around (and of course work) wearing only their shirts, which had long tails (down to about the knees) and were considered underwear. A man sans-culottes (spelled without a final S in the original), in the parlance of the time, was "naked."