Tip: If you are serious about improving your vocabulary, then Denise and I highly recommend you try the popular vocabulary-improvement software Ultimate Vocabulary. Click Ultimate Vocabulary for details.*
caballine, adjective, sounds soothing; it even sounds a bit like the name of a moisturising cream. But you wouldn't want someone to say you look "caballine". It means "horselike".
contrectation, noun. You might be good at concentration, but if someone says you are good at "contrectation", it means you are good at touching or fingering someone without their consent.
dentiloquist, noun, isn't as skilled a person as the sound of the word suggests; a "dentiloquist" is someone who talks through clenched teeth. (My mum would be a dentiloquist, but her teeth keep falling out.)
farraginous, adjective, is a fabulous word that means hotchpotch (or someone who makes a hotchpotch of something), much like this farraginous list of words. I, myself, am constantly being called farraginous — or something like that.
gnathonic, adjective, sounds nice because it resembles "Nathan"; and all Nathans I know are delightful. Yet "gnathonic" could hardly mean anything more different from Nathan-ic; "gnathonic" means "parasitical or deceitful".
indocible, adjective, sounds like a docile word; but it actually means "incapable of being taught or instructed".
jumentous, adjective, sounds so jubilant and momentous. But you really don't want to be called "jumentous"; it would mean you smell like horse wee.
molendinaceous, adjective, isn't really much of an insult, unless perhaps you have some peculiar anatomical feature; it means "resembling a windmill".
pyknic, adjective, is no picnic. It means "of or relating to a stocky physique with a rounded body and head, thickset trunk, and a tendency to fat" (Dr Who fans should think Sontaran.)
vecordious, adjective, isn't a very cordial description. It means senseless or lunatic.
On that vecordious note, this list of insulting words that sound innocent shows the benefits of a good vocabulary. If you can command words, then you can command knowledge and influence people. To develop your vocabulary, try the popular vocabulary-building software Ultimate Vocabulary.
I found all the above words in Peter Bowler, The Superior Person's Second Book of Weird and Wondrous Words (2003) and read the definitions in the Oxford English Dictionary Online.
*Denise and I recommend only products that we have tried and tested. These include Ultimate Vocabulary. We have agreed to receive a commission from some sales of Ultimate Vocabulary because we are happy to endorse that software.