trifler

from WordNet (r) 3.0 (2006)

trifler
    n 1: one who behaves lightly or not seriously

from The Collaborative International Dictionary of English v.0.48

Trifler \Tri"fler\, n.
   One who trifles. --Waterland.
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

63 Moby Thesaurus words for "trifler":
      Sunday painter, amateur, bungler, clock watcher, coquet,
      criticaster, dabbler, dabster, dallier, dawdle, dawdler, diddler,
      dilettante, dillydallier, disregarder, do-nothing, dodger,
      dolittle, doodler, faineant, flirt, fribble, gentleman of leisure,
      goldbrick, goldbricker, goof-off, grammaticaster, half scholar,
      idler, ignorer, laggard, lingerer, loafer, loiterer, loller,
      lotus-eater, lounger, lubber, malingerer, mope, moper, neglecter,
      negligent, philologaster, philosophaster, piddler, potterer,
      procrastinator, putterer, sciolist, shirker, slacker, slouch,
      sloucher, sloven, slug, sluggard, slut, smatterer,
      stick-in-the-mud, time killer, time waster, waiter on Providence