The Collaborative International Dictionary of English v.0.48:

Rapscallion \Rap*scal"lion\ (r[a^]p*sk[a^]l"y[u^]n), n. [See
   Rascallion.]
   A rascal; a good-for-nothing fellow. [Colloq.] --Howitt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rapscallion
    n 1: a deceitful and unreliable scoundrel [syn: rogue,
         knave, rascal, rapscallion, scalawag, scallywag,
         varlet]
    2: one who is playfully mischievous [syn: imp, scamp,
       monkey, rascal, rapscallion, scalawag, scallywag]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "rapscallion":
   bad boy, blackguard, booger, buffoon, bugger, cutup, devil, elf,
   enfant terrible, funmaker, hood, hoodlum, hooligan, imp, joker,
   jokester, knave, limb, little devil, little monkey, little rascal,
   minx, mischief, mischief-maker, pixie, practical joker, prankster,
   precious rascal, puck, rascal, rogue, rowdy, ruffian, scalawag,
   scamp, scapegrace, scoundrel, shyster, sneak, spalpeen, villain,
   wag