The Collaborative International Dictionary of English v.0.48:

Infrequent \In*fre"quent\, a. [L. infrequens : cf. F.
   infrequent. See In- not, and Frequent.]
   Seldom happening or occurring; rare; uncommon; unusual.
   [1913 Webster]

         The act whereof is at this day infrequent or out of use
         among all sorts of men.                  --Sir T.
                                                  Elyot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infrequent
    adj 1: not frequent; not occurring regularly or at short
           intervals; "infrequent outbursts of temper" [ant:
           frequent]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "infrequent":
   almost unheard-of, cheeseparing, chinchy, chintzy, exiguous,
   meager, miserly, niggardly, piddling, poor, rare, scant, scanty,
   scarce, scattered, scrimping, scrimpy, seldom met with,
   seldom seen, skimping, skimpy, slim, slow, sparse, spotty,
   sprinkled, stingy, thin, tight, uncommon, unfrequent, unique,
   unusual