The Collaborative International Dictionary of English v.0.48:

Flimsy \Flim"sy\, a. [Compar. Flimsier; superl. Flimsiest.]
   [Cf. W. llymsi naked, bare, empty, sluggish, spiritless. Cf.
   Limsy.]
   Weak; feeble; limp; slight; vain; without strength or
   solidity; of loose and unsubstantial structure; without
   reason or plausibility; as, a flimsy argument, excuse,
   objection.
   [1913 Webster]

         Proud of a vast extent of flimsy lines.  --Pope.
   [1913 Webster]

         All the flimsy furniture of a country miss's brain.
                                                  --Sheridan.

   Syn: Weak; feeble; superficial; shallow; vain.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flimsy \Flim"sy\, n.
   1. Thin or transfer paper.
      [1913 Webster]

   2. A bank note. [Slang, Eng.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

flimsy
    adj 1: lacking solidity or strength; "a flimsy table"; "flimsy
           construction"
    2: not convincing; "unconvincing argument"; "as unconvincing as
       a forced smile" [syn: unconvincing, flimsy] [ant:
       convincing]
    3: lacking substance or significance; "slight evidence"; "a
       tenuous argument"; "a thin plot"; a fragile claim to fame"
       [syn: flimsy, fragile, slight, tenuous, thin]
    n 1: a thin strong lightweight translucent paper used especially
         for making carbon copies [syn: onionskin, flimsy]

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "flimsy":
   adulterated, airy, asinine, attenuate, attenuated, boyish,
   breakable, brittle, brittle as glass, catchpenny, cheap-jack,
   cobwebby, crackable, crisp, crispy, crumbly, crushable, cut,
   dainty, decrepit, delicate, delicately weak, diaphanous,
   dilapidated, dilute, diluted, effeminate, empty, ethereal, fatuous,
   feeble, filmy, fine, fine-drawn, finespun, fissile, flabby,
   flaccid, floppy, foolish, fracturable, fragile, frail, frangible,
   friable, fribble, fribbling, frivolous, frothy, futile, gaseous,
   gauzy, gimcrack, gimcracky, girlish, gossamer, gossamery, gracile,
   idle, implausible, improbable, inadequate, inane, inconceivable,
   inconclusive, incredible, infirm, insubstantial, jerry,
   jerry-built, lacerable, lacy, lax, light, lightweight, limp, loose,
   makeshift, misty, namby-pamby, nugacious, nugatory, otiose, paltry,
   papery, pasteboardy, poor, puny, ramshackle, rare, rarefied,
   relaxed, rickety, rubbery, scissile, shaky, shallow, shatterable,
   shattery, sheer, shivery, silly, sissified, sleazy, slender,
   slenderish, slight, slight-made, slim, slimmish, slinky, small,
   splintery, subtile, subtle, superficial, svelte, sylphlike, tacky,
   tenuous, thin, thin-bodied, thin-set, thin-spun, thinned,
   thinned-out, thinnish, threadlike, transparent, trifling, trite,
   trivial, unbelievable, uncompact, uncompressed, unconvincing,
   unproved, unrigorous, unsatisfactory, unsound, unsubstantial,
   unsustained, vacuous, vague, vain, vapid, vaporous, vulnerable,
   wasp-waisted, watered, watered-down, watery, weak, weakly, willowy,
   windy, wiredrawn, wispy, womanish