The Collaborative International Dictionary of English v.0.48:

Incessantly \In*ces"sant*ly\, adv.
   Unceasingly; continually. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incessantly
    adv 1: with unflagging resolve; "dance inspires him ceaselessly
           to strive higher and higher toward the shining pinnacle
           of perfection that is the goal of every artiste" [syn:
           endlessly, ceaselessly, incessantly, unceasingly,
           unendingly, continuously]
    2: without interruption; "the world is constantly changing"
       [syn: constantly, always, forever, perpetually,
       incessantly]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "incessantly":
   ad infinitum, again and again, always, at a stretch, at all times,
   ceaselessly, connectedly, constantly, continually, continuously,
   cumulatively, cyclically, daily, daily and hourly, day after day,
   day and night, endlessly, eternally, ever, ever and anon,
   everlastingly, every day, every hour, every moment,
   hour after hour, hourly, in perpetuity, indestructibly, infinitely,
   interminably, monotonously, month after month, never-endingly,
   night and day, on a stretch, on and on, on end, perdurably,
   perennially, permanently, perpetually, rapidly, regularly,
   repetitively, right along, round the clock, steadily, sustainedly,
   time without end, together, unbrokenly, unceasingly, unendingly,
   unintermittently, uninterruptedly, unrelievedly, unvaryingly,
   without a break, without cease, without end, without letup,
   without stopping, world without end, year after year