What is "left-handed screw"

Left-handed \Left"-hand`ed\, a.

1. Having the left hand or arm stronger and more dexterous than the right; using the left hand and arm with more dexterity than the right.

2. Clumsy; awkward; unlucky; insincere; sinister; malicious; as, a left-handed compliment.

   The commendations of this people are not always left-handed and detractive.
   --Landor.

3. Having a direction contrary to that of the hands of a watch when seen in front; -- said of a twist, a rotary motion, etc., looked at from a given direction.

   Left-handed marriage, a morganatic marriage. See Morganatic.

   Left-handed screw, a screw constructed to advance away from the observer, when turned, as in a nut, with a left-handed rotation. An ordinary wood screw is right-handed.