The Collaborative International Dictionary of English v.0.48:

Insufficient \In`suf*fi"cient\, a. [L. insufficiens, -entis. See
   In- not, and Sufficient.]
   [1913 Webster]
   1. Not sufficient; not enough; inadequate to any need, use,
      or purpose; as, the provisions are insufficient in
      quantity, and defective in quality. "Insufficient for His
      praise." --Cowper.
      [1913 Webster]

   2. Wanting in strength, power, ability, capacity, or skill;
      incompetent; incapable; unfit; as, a person insufficient
      to discharge the duties of an office.

   Syn: Inadequate; scanty; incommensurate; unequal; unfit;
        incompetent; incapable; inefficient.
        [1913 Webster]

WordNet (r) 3.0 (2006):

insufficient
    adj 1: of a quantity not able to fulfill a need or requirement;
           "insufficient funds" [syn: insufficient, deficient]
           [ant: sufficient]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "insufficient":
   asymmetric, base, defective, deficient, disappointing,
   disheartening, disparate, displeasing, disproportionate,
   dissatisfactory, failing, found wanting, half-assed, ill-matched,
   ill-sorted, imperfect, inadequate, incommensurate, incompetent,
   incomplete, inferior, irregular, lacking, little, maladroit, mean,
   mediocre, minus, mismatched, not comparable, not enough,
   not good enough, not in it, odd, out of it, out of proportion,
   petty, scant, scanty, scarce, shabby, short, short of, shy, skew,
   skewed, small, too little, trivial, unequal, unequal to, uneven,
   unfulfilling, ungratifying, unqualified, unreached, unsatisfactory,
   unsatisfying, unskillful, unsufficing, wanting