The Collaborative International Dictionary of English v.0.48:

Idle \I"dle\, a. [Compar. Idler; superl. Idlest.] [OE. idel,
   AS. [imac]del vain, empty, useless; akin to OS. [imac]dal, D.
   ijdel, OHG. [imac]tal vain, empty, mere, G. eitel, Dan. & Sw.
   idel mere, pure, and prob. to Gr. ? clear, pure, ? to burn.
   Cf. Ether.]
   1. Of no account; useless; vain; trifling; unprofitable;
      thoughtless; silly; barren. "Deserts idle." --Shak.
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            Every idle word that men shall speak, they shall
            give account thereof in the day of judgment. --Matt.
                                                  xii. 36.
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            Down their idle weapons dropped.      --Milton.
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            This idle story became important.     --Macaulay.
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   2. Not called into active service; not turned to appropriate
      use; unemployed; as, idle hours.
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            The idle spear and shield were high uphing.
                                                  --Milton.
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   3. Not employed; unoccupied with business; inactive; doing
      nothing; as, idle workmen.
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            Why stand ye here all the day idle?   --Matt. xx. 6.
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   4. Given rest and ease; averse to labor or employment; lazy;
      slothful; as, an idle fellow.
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   5. Light-headed; foolish. [Obs.] --Ford.
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   Idle pulley (Mach.), a pulley that rests upon a belt to
      tighten it; a pulley that only guides a belt and is not
      used to transmit power.

   Idle wheel (Mach.), a gear wheel placed between two others,
      to transfer motion from one to the other without changing
      the direction of revolution.

   In idle, in vain. [Obs.] "God saith, thou shalt not take
      the name of thy Lord God in idle." --Chaucer.

   Syn: Unoccupied; unemployed; vacant; inactive; indolent;
        sluggish; slothful; useless; ineffectual; futile;
        frivolous; vain; trifling; unprofitable; unimportant.

   Usage: Idle, Indolent, Lazy. A propensity to inaction
          is expressed by each of these words; they differ in
          the cause and degree of this characteristic. Indolent
          denotes an habitual love to ease, a settled dislike of
          movement or effort; idle is opposed to busy, and
          denotes a dislike of continuous exertion. Lazy is a
          stronger and more contemptuous term than indolent.
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WordNet (r) 3.0 (2006):

idle pulley
    n 1: a pulley on a shaft that presses against a guide belt to
         guide or tighten it [syn: idle pulley, idler pulley,
         idle wheel]