WordNet (r) 3.0 (2006):

weirdo
    n 1: someone unpleasantly strange or eccentric [syn: creep,
         weirdo, weirdie, weirdy, spook]
    2: someone deranged and possibly dangerous [syn: crazy,
       loony, looney, nutcase, weirdo]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "weirdo":
   aliene, bedlamite, borderline case, character, crackbrain,
   crackpot, crank, dement, demoniac, eccentric, energumen, erratic,
   fanatic, flake, fou, freak, idiot, kook, loon, loony, lunatic,
   madman, maniac, meshuggenah, noncompos, nonconformist, nut,
   oddball, phrenetic, raving lunatic, recluse, screwball,
   strange duck