The Collaborative International Dictionary of English v.0.48:

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

WordNet (r) 3.0 (2006):

trifler
    n 1: one who behaves lightly or not seriously

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