The Collaborative International Dictionary of English v.0.48:

Endlessly \End"less*ly\, adv.
   In an endless manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

endlessly
    adv 1: continuing forever without end; "there are infinitely
           many possibilities" [syn: infinitely, endlessly]
           [ant: finitely]
    2: 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]
    3: (spatial sense) without bounds; "the Nubian desert seemed to
       stretch out before them endlessly"
    4: all the time; seemingly without stopping; "a theological
       student with whom I argued interminably"; "her nagging went
       on endlessly" [syn: interminably, endlessly]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "endlessly":
   ad infinitum, again and again, at a stretch, boresomely, boringly,
   boundlessly, ceaselessly, connectedly, constantly, continually,
   continuously, cumulatively, cyclically, drearisomely, dully,
   eternally, everlastingly, fatiguingly, forever, harpingly,
   illimitably, immeasurably, immensely, in perpetuity, incalculably,
   incessantly, incomprehensibly, indestructibly, infinitely,
   innumerably, interminably, irksomely, limitlessly, measurelessly,
   monotonously, never-endingly, on a stretch, on and on, on end,
   perdurably, perennially, permanently, perpetually, repetitively,
   round the clock, steadily, tediously, time without end, tiresomely,
   to infinity, together, unbrokenly, unceasingly, unendingly,
   unintermittently, uninterruptedly, unrelievedly, unvaryingly,
   wearisomely, wearyingly, without a break, without cease,
   without end, without limit, without stopping, world without end