Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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