Yesterday some friends and I got into a debate about what the opposite of "ambidextrous" would be. Monodextrous? Solodextrous? Just plain dexterous? It turns out we were all wrong. See, we were operating under the erroneous assumption that the "-dextrous" part of the word had something to do with skill or proficiency — you know, like dexterity.
Then I looked it up.
Apparently the "-dextrous" portion of the word actually means "right-handed," so ambidexterity is the ability to use both hands as though they were your right hand — a definition that belies the odd ways in which right-handedness continues to be embedded as a virtue in our society. In other words, the assumption is that folks can only use their right hands skillfully, so being ambidextrous is like a weird opposite of having two left feet.
So what is the actual antonym of ambidextrous? The literal (and archaic) word is ambilevous — a term from centuries ago that literally means both hands are left hands, and was therefore used to indicate that one was awkward or clumsy with their hands. (A more direct correlative for having two left feet.) But that word is no longer in use. The contemporary antonym of ambidextrous is ambisinister. Yup, ambisinister. It means equally clumsy or unskillful with both hands. Just as the Latin root "dexter" means right (the direction), but came to connote favorability and morality, so the Latin root "sinistra" means left, but came to connote evil and ill luck. (In heraldry, the terms dexter and sinister echo these larger definitional trends while maintaining the sense of direction we often associate with right and left today; there's a great bit about this on Wikipedia you can check out here.)
So there you have it. You can either be right-handed, left-handed, ambidextrous, or ambisinister (ambisinistrous?). Proof positive that words continue to be both fascinating and revealing.