The Collaborative International Dictionary of English v.0.48:

Unsystematic \Unsystematic\
   See systematic.

WordNet (r) 3.0 (2006):

unsystematic
    adj 1: lacking systematic arrangement or method or organization;
           "unsystematic and fragmentary records"; "he works in an
           unsystematic manner" [ant: systematic]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "unsystematic":
   aimless, amorphous, broken, capricious, careening, casual, catchy,
   changeable, changing, choppy, desultory, deviating, deviative,
   deviatory, different, disarticulated, disconnected, discontinuous,
   disjunct, disordered, disorderly, dispersed, disproportionate,
   divaricate, divergent, diversified, diversiform, eccentric, erose,
   erratic, fitful, flickering, fluctuating, formless, frivolous,
   gratuitous, guttering, halting, haphazard, herky-jerky,
   heteroclite, hit-or-miss, immethodical, impulsive, inchoate,
   incoherent, inconsistent, inconstant, indiscriminate, intermittent,
   intermitting, irregular, jagged, jerky, lurching, meaningless,
   mercurial, misshapen, motley, mutable, nonconformist, nonstandard,
   nonsymmetrical, nonsystematic, nonuniform, orderless, patchy,
   planless, pluralistic, promiscuous, ragged, rambling, random,
   rough, scrappy, senseless, shapeless, snatchy, spasmatic, spasmic,
   spasmodic, spastic, sporadic, spotty, staggering, straggling,
   straggly, systemless, unarranged, uncertain, unclassified,
   undirected, unequable, unequal, uneven, ungraded, unjoined,
   unmethodical, unmetrical, unordered, unorganized, unorthodox,
   unregular, unrhythmical, unsettled, unsorted, unstable, unsteady,
   unsymmetrical, ununiform, vague, variable, variegated, variform,
   various, varying, veering, wandering, wavering, wobbling, wobbly