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.