Ambisinister \am-bi-SIN-uh-ster\ , adjective;
1. Clumsy or unskillful with both hands
This word isn't in the online OED, which is weird, so let's talk about its component parts instead.
Sinister dates to the early 15th century meaning "prompted by malice or ill-will". It comes from Old French sinistre ("contrary, unfavorable, to the left") which derives from Latin sinister ("left, on the left side"), the opposite of dexter. The etymology of the Latin word is not completely certain, but there are a couple of theories: it may be from the base *sen- ("the slower or weaker hand") or might be a euphemism connected with the root of Sanskrit saniyan ("more useful, more advantageous"). It was used in the sense of "unlucky, unfavorable" because an seeing an omen to one's left was regarded as signaling bad luck or misfortune. This is where the sense of "harmful, unfavorable, adverse" came from. Interestingly, the left was the favorable side in traditional Roman auspices, but the influence of Greeks switched that sentiment. The prefix ambi- is a combining form meaning "both, on both sides" from Latin ambi- ("around, round about") which derives from Proto-Indo-European *ambhi- ("around"). The Proto-Indo-Europen root was probably an ablative plural of *ant-bhi ("from both sides") which comes from *ant- ("front, forehead").
So ambisinister is something like "both left" so this word is the manual answer to "two left feet": "having two left hands" or "clumsy with both hands".
As a side note: Mr. B guessed the meaning of this word immediately because he is left-handed and apparently lefties are completely aware of the meaning of sinister. So the question is, would an ambisinister lefty be highly dexterous rather than clumsy/unskillful? I wonder...
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Tuesday, February 1
Etymologies come from the Oxford English Dictionary and/or Etymonline.com