I had been under the impression that the associations went the other way - that the Latin Dexter came from the Indo-European as did the Latin Sinestra. From those, we derived the association of the right hand with dexterity (so a person with 2 right hands is ambidextrous whereas a person with 2 left feet is clumsy) and the sneaky connotations of "sinister" (as you watch the person's right hand to ensure he will not attack you; a sinister person might stab with the left hand by surprise).
I just read a blog post on NeuroLogica Blog that could have been a LW post, so I figured that I would bring it on over. It basically details how knowing about our biases can help us correct for them, a la the lens that sees its flaws, and then brings to light a new study (unfortunately behind a paywall... I wanted to see the methodology) that shows that the simple act of wearing a glove on your dominant hand can influence how you perceive the world.
When I learned that Dexter and Sinister were Latin words for Right and Left, respectively, I was told that it came from shield formations, and how the person on your left was a leech for using your shield protection, and the one on your right was your protector. Now that explanation sounds a bit hollow.