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.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Idler \I"dler\, n.
   1. One who idles; one who spends his time in inaction; a lazy
      person; a sluggard.
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   2. (Naut.) One who has constant day duties on board ship, and
      keeps no regular watch. --Totten.
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   3. (Mach.) An idle wheel or pulley. See under Idle. Idless

WordNet (r) 3.0 (2006):

idler
    n 1: person who does no work; "a lazy bum" [syn: idler,
         loafer, do-nothing, layabout, bum]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "idler":
   Arab, beach bum, beachcomber, beggar, bo, bum, bummer, bungler,
   clock watcher, dallier, dawdle, dawdler, diddler, dillydallier,
   disregarder, do-nothing, dodger, dogie, dolittle, doodler, drone,
   faineant, gamin, gamine, gentleman of leisure, goldbrick,
   goldbricker, goof-off, guttersnipe, hobo, homeless waif, ignorer,
   laggard, landloper, lazybones, lazzarone, lingerer, loafer,
   loiterer, loller, losel, lotus-eater, lounge lizard, lounger,
   lubber, malingerer, mope, moper, mudlark, neglecter, negligent,
   piker, potterer, procrastinator, putterer, ragamuffin, ragman,
   ragpicker, rounder, shirker, ski bum, slacker, slouch, sloucher,
   sloven, slug, slugabed, sluggard, slut, stick-in-the-mud, stiff,
   stray, street Arab, street urchin, sundowner, surf bum, swagman,
   swagsman, tatterdemalion, tennis bum, time killer, time waster,
   tramp, trifler, turnpiker, urchin, vag, vagabond, vagrant, waif,
   waifs and strays, waiter on Providence, wastrel