The Collaborative International Dictionary of English v.0.48:

Erratic \Er*rat"ic\, a. [L. erraticus, fr. errare to wander: cf.
   F. erratique. See Err.]
   1. Having no certain course; roving about without a fixed
      destination; wandering; moving; -- hence, applied to the
      planets as distinguished from the fixed stars.
      [1913 Webster]

            The earth and each erratic world.     --Blackmore.
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   2. Deviating from a wise of the common course in opinion or
      conduct; eccentric; strange; queer; as, erratic conduct.
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   3. Irregular; changeable. "Erratic fever." --Harvey.
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   Erratic blocks, gravel, etc. (Geol.), masses of stone
      which have been transported from their original resting
      places by the agency of water, ice, or other causes.

   Erratic phenomena, the phenomena which relate to
      transported materials on the earth's surface.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Erratic \Er*rat"ic\, n.
   1. One who deviates from common and accepted opinions; one
      who is eccentric or preserve in his intellectual
      character.
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   2. A rogue. [Obs.] --Cockeram.
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   3. (Geol.) Any stone or material that has been borne away
      from its original site by natural agencies; esp., a large
      block or fragment of rock; a bowlder.
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   Note: In the plural the term is applied especially to the
         loose gravel and stones on the earth's surface,
         including what is called drift.
         [1913 Webster]

WordNet (r) 3.0 (2006):

erratic
    adj 1: liable to sudden unpredictable change; "erratic
           behavior"; "fickle weather"; "mercurial twists of
           temperament"; "a quicksilver character, cool and willful
           at one moment, utterly fragile the next" [syn: erratic,
           fickle, mercurial, quicksilver(a)]
    2: having no fixed course; "an erratic comet"; "his life
       followed a wandering course"; "a planetary vagabond" [syn:
       erratic, planetary, wandering]
    3: likely to perform unpredictably; "erratic winds are the bane
       of a sailor"; "a temperamental motor; sometimes it would
       start and sometimes it wouldn't"; "that beautiful but
       temperamental instrument the flute"- Osbert Lancaster [syn:
       erratic, temperamental]

Moby Thesaurus II by Grady Ward, 1.0:

271 Moby Thesaurus words for "erratic":
   aberrant, aberrative, abnormal, adrift, afloat, agnostic, aimless,
   alternating, ambiguous, amorphous, anomalistic, anomalous, bizarre,
   broken, capricious, careening, casual, catchy, chancy, changeable,
   changeful, changing, character, choppy, circuitous, crackpot,
   crank, crankish, cranky, crotchety, curving, departing, desultory,
   deviable, deviant, deviating, deviative, deviatory, devious, dicey,
   different, digressive, directionless, disarticulated, disconnected,
   discontinuous, discursive, disjunct, disordered, disorderly,
   dispersed, disproportionate, divagatory, divaricate, divergent,
   diversified, diversiform, dizzy, dotty, doubtful, doubting,
   dubious, eccentric, equivocal, erose, errant, exceptional,
   excursive, extraordinary, fast and loose, fey, fickle, fitful,
   flake, flaky, flickering, flighty, flitting, fluctuant,
   fluctuating, formless, freak, freakish, frivolous, funny, giddy,
   gratuitous, guttering, halting, haphazard, herky-jerky, hesitant,
   hesitating, heteroclite, heteromorphic, hit-or-miss, idiocratic,
   idiosyncratic, iffy, immethodical, impetuous, impulsive,
   incalculable, inchoate, incoherent, inconsistent, inconstant,
   indecisive, indemonstrable, indirect, indiscriminate, infirm,
   intermittent, intermitting, irregular, irresolute, irresponsible,
   jagged, jerky, kinky, kook, kooky, labyrinthine, lurching, maggoty,
   mazy, meandering, meaningless, mercurial, misshapen, moody, motley,
   mutable, nonconformist, nonstandard, nonsymmetrical, nonsystematic,
   nonuniform, nut, nutty, odd, oddball, orderless, out-of-the-way,
   outlandish, patchy, peculiar, planetary, planless, pluralistic,
   polysemous, promiscuous, quaint, queer, quirky, ragged, rambling,
   random, recluse, restless, rough, roundabout, roving,
   scatterbrained, scrappy, screwball, screwy, senseless, serpentine,
   shapeless, shifting, shifty, shuffling, singular, skeptical, snaky,
   snatchy, spasmatic, spasmic, spasmodic, spastic, spineless,
   sporadic, spotty, staggering, straggling, straggly, strange,
   strange duck, stray, straying, subnormal, swerving, systemless,
   touch-and-go, turning, twisted, twisting, unaccountable,
   unarranged, uncertain, unclassified, unconfirmable, uncontrolled,
   unconventional, unconvinced, undependable, undirected,
   undisciplined, undivinable, unequable, unequal, uneven, unfixed,
   unforeseeable, ungraded, unjoined, unmethodical, unmetrical,
   unnatural, unordered, unorganized, unorthodox, unpersuaded,
   unpredictable, unprovable, unregular, unreliable, unrestrained,
   unrhythmical, unsettled, unsorted, unstable, unstable as water,
   unstaid, unsteadfast, unsteady, unsure, unsymmetrical,
   unsystematic, ununiform, unusual, unverifiable, vacillating,
   vagarious, vagrant, vague, variable, variegated, variform, various,
   varying, veering, vicissitudinary, vicissitudinous, volatile,
   wacky, wandering, wanton, wavering, wavery, wavy, wayward, weird,
   weirdo, whimsical, winding, wishy-washy, wobbling, wobbly,
   zigzag