Моветон - from the French "mauvais ton," or "bad taste."
I came across an interesting use of the word in the Jan. 25 issue of Dengi. The context is a discussion of the changing role of in-house security employees at Russian companies, specifically the fact that they are less likely to kidnap people now than they once were:
"Подобные ситуации были широко распространены в "лихие девяностые", когда милиция практически не работала по защите "проклятых буржуев". В настоящее время это уже моветон. Но главное не в приличиях, а в неэффективности и рискованности таких действий."
My translation:
"Things like that [kidnappings] were a common occurrence in the wild 90s because the police were basically refusing to protect 'bourgeois pigs.' That would be mauvais ton today. And it's not really a question of manners. It's just ineffective and risky."
I thought it was interesting that the interviewee used the term "mauvais ton" and immediately followed it with the qualification that he isn't talking about manners. He seems to be using моветон to mean "stupid."
When I turned to the Russian National Corpus and searched for моветон, I found this whimsical line from a play by Maxim Gorky:
Не гризе па ле семиачки, се моветон.
Don't munch on les sunflower seeds, c'est mauvais ton.