Did you not find an answer yourself; i.e. 'asshole'? Most other anatomical inserts also apply, at least the non-gendered ones.
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Does not offend any particular group, except perhaps those with a colostomy.
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Still an insult.
The remainder are generally insulting because they imply you're so X that you must be a member of group X. I.e. calling someone brain-dead, an idiot, a moron, slow, the French word for slow etc. You can't call someone stupid without calling them stupid, even in an indirect way.
Edit: Neanderthal and troglodyte might work on the grounds that you're comparing them to extinct species, and of course you could go for inanimate objects e.g. thick as pigshit, as smart as a bag of rocks etc.
Manuals can still be faster if you know you're about to put your foot down, and you're in the cheaper end of the market.