The Collaborative International Dictionary of English v.0.48:

Ad infinitum \Ad in`fi*ni"tum\ [L., to infinity.]
   Without limit; endlessly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ad infinitum
    adv 1: to infinity; without or seemingly without limit; "talked
           on and on ad infinitum"

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "ad infinitum":
   again and again, all over, at a stretch, at all points,
   at full length, at length, boundlessly, ceaselessly, connectedly,
   constantly, continually, continuously, cumulatively, cyclically,
   endlessly, eternally, everlastingly, every bit, every inch,
   every whit, extensively, forever, head and shoulders,
   heart and soul, illimitably, immeasurably, immensely,
   in all respects, in every respect, in extenso, in perpetuity,
   incalculably, incessantly, incomprehensibly, indestructibly,
   infinitely, innumerably, inside and out, interminably, lengthily,
   limitlessly, measurelessly, monotonously, neck deep,
   never-endingly, on a stretch, on all counts, on and on, on end,
   overall, perdurably, perennially, permanently, perpetually,
   repetitively, root and branch, round the clock, steadily,
   through and through, throughout, time without end, to infinity,
   to the brim, to the death, to the end, to the hilt, together,
   unbrokenly, unceasingly, unendingly, unintermittently,
   uninterruptedly, unrelievedly, without a break, without cease,
   without end, without limit, without stopping, world without end,
   you name it