The Collaborative International Dictionary of English v.0.48:

Threadbare \Thread"bare`\, a.
   1. Worn to the naked thread; having the nap worn off;
      threadbare clothes. "A threadbare cope." --Chaucer.
      [1913 Webster]

   2. Fig.: Worn out; as, a threadbare subject; stale topics and
      threadbare quotations. --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

threadbare
    adj 1: repeated too often; overfamiliar through overuse;
           "bromidic sermons"; "his remarks were trite and
           commonplace"; "hackneyed phrases"; "a stock answer";
           "repeating threadbare jokes"; "parroting some timeworn
           axiom"; "the trite metaphor `hard as nails'" [syn:
           banal, commonplace, hackneyed, old-hat,
           shopworn, stock(a), threadbare, timeworn,
           tired, trite, well-worn]
    2: having the nap worn away so that the threads show through;
       "threadbare rugs"

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "threadbare":
   back-number, banal, bare, bathetic, bewhiskered, boring, bromidic,
   cliche, cliche-ridden, cliched, common, commonplace, corny,
   cut-and-dried, damaged, deep-worn, dilapidated, dingy, dog-eared,
   down-at-heel, dull, fade, faded, familiar, frayed, fusty, hack,
   hackney, hackneyed, imitative, impaired, injured, monotonous,
   moth-eaten, musty, old hat, overused, overworked, platitudinous,
   played out, prosaic, ragged, run-down, scruffy, seedy, sere, set,
   shabby, shelfworn, shopworn, slovenly, sorry, square, stale,
   stereotyped, stock, tacky, tattered, tatty, tedious, timeworn,
   tired, tiresome, torn, trite, truistic, uncreative, unoriginal,
   warmed-over, well-known, well-worn, worn, worn out, worn ragged,
   worn thin, worn to rags, worn to threads, worn-down, worn-out,
   wretched