The Collaborative International Dictionary of English v.0.48:

Scatter-brained \Scat"ter-brained`\, a.
   Giddy; thoughtless.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Shatter-brained \Shat"ter-brained`\, Shatter-pated
\Shat"ter-pat`ed\, a.
   Disordered or wandering in intellect; hence, heedless; wild;
   similar to scatter-brained. --J. Goodman.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

scatterbrained
    adj 1: lacking sense or discretion; "his rattlebrained crackpot
           ideas"; "how rattlepated I am! I've forgotten what I came
           for"- Glenway Westcott [syn: rattlebrained,
           rattlepated, scatterbrained, scatty]
    2: guided by whim and fancy; "flighty young girls" [syn:
       flighty, flyaway, head-in-the-clouds, scatterbrained]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "scatterbrained":
   adrift, afloat, alternating, amorphous, blank, brainless,
   capricious, changeable, changeful, dazed, desultory, deviable,
   dippy, dizzy, dopey, eccentric, empty, empty-headed, empty-minded,
   empty-pated, empty-skulled, erratic, fast and loose,
   featherbrained, featherheaded, fickle, fitful, flibbertigibbet,
   flickering, flighty, flitting, fluctuating, fluttery, freakish,
   frivolous, gaga, giddy, giddy-brained, giddy-headed, giddy-pated,
   giddy-witted, harebrain, harebrained, hollow, impetuous, impulsive,
   inane, inconsistent, inconstant, indecisive, infirm, irregular,
   irresolute, irresponsible, jejune, mazy, mercurial, moody,
   rambling, rattlebrained, rattleheaded, rattlepated, restless,
   roving, scramblebrained, shapeless, shatterbrained, shifting,
   shifty, shuffling, silly, spasmodic, spineless, thoughtless,
   unaccountable, uncertain, uncontrolled, undependable,
   undisciplined, unfixed, unpredictable, unreliable, unrestrained,
   unsettled, unstable, unstable as water, unstaid, unsteadfast,
   unsteady, vacant, vacillating, vacuous, vagrant, vapid, variable,
   vicissitudinary, vicissitudinous, volatile, wandering, wanton,
   wavering, wavery, wavy, wayward, whimsical, wishy-washy, witless