The Collaborative International Dictionary of English v.0.48:

Infrequently \In*fre"quent*ly\, adv.
   Not frequently; rarely.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infrequently
    adv 1: not many times; "in your 1850 church you not infrequently
           find a dramatic contrast between the sumptuous
           appointments of the building itself and the inhuman
           barrack-like living conditions in the church room" [ant:
           frequently, oft, often, oftentimes, ofttimes]