I don't mean euphemisms or backhanded compliments (or the reverse). It's the words which sound grand, but actually have a bad meaning (e.g., jumentous: smelling strongly like a beast of burden). Is there any word to define this conceptually?
Sesquipedalianism is the habit of using long (sesquipedalian, or "foot-and-a-half") words; it doesn't specifically mean "using long words in order to disguise insults", but a few people I know who are habitual sesquipedalianists1 do so for exactly that reason.
Obscurantism is "deliberate obscurity or evasion of clarity"; if you habitually use long words to hide your true meaning, you're an obscurantist.
I can't think of any word that combines those meanings with the specific intent to hide insults.
1 This word doesn't seem to actually exist yet, so I'm coining it. "Sesquipedalian" is the adjective form, but it refers to the words themselves, not the people who [mis]use them.
- Brave (often means stupid: "That's brave of you!")
- Interesting (can mean boring: "How very interesting." or stupid: "An interesting idea. Why don't you present that at the next staff meeting?")
- Special (its sense of developmentally disabled was originally meant as a polite euphemism, as in the Special Olympics or special needs... but because of that association, one can sneer at anything by calling it "special", while appearing to praise it. "Oh, did your kid make that mug for you? Well, isn't that special?")
- Great personality (when you're being set up for a blind date, if your friend tells you that your date has "a great personality", it means s/he is ugly. Or at least that's the cliché.)
I'd say 'ostentatious' sounds quite grand, but is generally a bit of an insult.
An insult that can sound like a compliment if you say it with a smile 'You are amazingly obsequious ". Sounds like a compliment but it really means you are an amazing brown noser