The Collaborative International Dictionary of English v.0.48:

screwball \screw"ball\, n.
   1. An eccentric or crazy person; an oddball.
      [PJC]

   2. (Baseball) A baseball pitch that curves in the direction
      opposite to that of a curve ball.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

screwball \screw"ball\, adj.
   1. Eccentric; zany; crazy; as, screwball antics; a screwball
      comedy.
      [PJC]

   2. Ridiculously unsound; improbable or doomed to failure; --
      of plans or ideas; as, a screwball plan to raise cocoanuts
      in Alaska.
      [PJC]

WordNet (r) 3.0 (2006):

screwball
    adj 1: foolish; totally unsound; "a crazy scheme"; "half-baked
           ideas"; "a screwball proposal without a prayer of
           working" [syn: crazy, half-baked, screwball,
           softheaded]
    n 1: a whimsically eccentric person [syn: crackpot, crank,
         nut, nut case, fruitcake, screwball]
    2: a pitch with reverse spin that curves toward the side of the
       plate from which it was thrown

Moby Thesaurus II by Grady Ward, 1.0:

157 Moby Thesaurus words for "screwball":
   abnormal, alien, aliene, anomalous, balmy, bananas, barmy, bats,
   batty, beany, bedlamite, bonkers, borderline case, bowl, buggy,
   bughouse, bugs, case, cast, change of pace, change-up, character,
   chuck, chunk, crackbrain, crackers, crackpot, crank, crankish,
   cranky, crotchety, cuckoo, curve, daffy, dement, demoniac, deviant,
   deviative, different, ding-a-ling, dippy, divergent, dotty,
   downcurve, eccentric, energumen, erratic, exceptional, fanatic,
   fastball, fey, flake, flaky, fling, flip, flipped, forward pass,
   fou, freak, freaked-out, freakish, fruitcakey, fruity, funny, gaga,
   goofy, harebrain, haywire, heave, hermit, hobo, hurl, idiocratic,
   idiosyncratic, idiot, incurve, irregular, just plain nuts, kinky,
   knuckleball, kook, kooky, lateral, lateral pass, lob, lone wolf,
   loner, loon, loony, loopy, lunatic, madman, maggoty, maniac,
   maverick, meshuggenah, natural, noncompos, nonconformist, nut,
   nuts, nutty, odd, odd fellow, oddball, oddity, off the hinges,
   off the track, off the wall, original, outcurve, outsider, pariah,
   pass, peculiar, peg, phrenetic, pitch, potty, put, queer,
   queer duck, queer fish, queer specimen, quirky, rara avis,
   raving lunatic, recluse, round the bend, screwballs, screwy, serve,
   service, shot-put, shy, singular, sinker, slaphappy, slider, sling,
   solitary, spitball, spitter, strange, strange duck, throw, toss,
   tramp, twisted, type, unconventional, unnatural, upcurve, wacky,
   weirdo, whimsical, zealot