The Collaborative International Dictionary of English v.0.48:

Riffraff \Riff"raff`\, n. [OE. rif and raf every particle, OF.
   rif et raf. CF. Raff, and 1st Rifle.]
   Sweepings; refuse; the lowest order of society. --Beau. & Fl.
   [1913 Webster]

WordNet (r) 3.0 (2006):

riffraff
    n 1: disparaging terms for the common people [syn: rabble,
         riffraff, ragtag, ragtag and bobtail]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "riffraff":
   bilge, bilgewater, canaille, carrion, cattle, chaff, clamjamfry,
   debris, dishwater, ditchwater, dregs, dregs of society, dust,
   garbage, junk, kelter, litter, lumber, masses, mob, offal,
   offscourings, offscum, proletariat, raff, ragtag and bobtail,
   refuse, rubbish, rubble, scrap, scum, scurf, sewage, sewerage,
   shoddy, slop, slops, slough, sordes, swill, swinish multitude,
   trash, truck, unwashed, vermin, waste