The Collaborative International Dictionary of English v.0.48:

Immeasurably \Im*meas"ur*a*bly\, adv.
   In an immeasurable manner or degree. "Immeasurably distant."
   --Wordsworth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immeasurably
    adv 1: to an immeasurable degree; beyond measurement; "the war
           left him immeasurably fearful of what man can do to man"
           [ant: measurably]
    2: without bounds; "he is infinitely wealthy" [syn:
       boundlessly, immeasurably, infinitely]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "immeasurably":
   a outrance, absolutely, abundantly, acervatim, ad infinitum,
   all out, beyond all bounds, beyond compare, beyond comparison,
   beyond measure, boundlessly, completely, copiously, countlessly,
   dead, downright, endlessly, essentially, eternally, extremely,
   flat out, forever, fundamentally, illimitably, immensely,
   in crowds, in heaps, in perpetuity, in the extreme, in throngs,
   incalculably, incomprehensibly, indefinitely, inexhaustibly,
   infinitely, innumerably, interminably, limitlessly, measurelessly,
   most, multitudinously, no end, numerously, perfectly, prodigally,
   profusely, purely, radically, thickly, to infinity, totally,
   unconditionally, unequivocally, utterly, with a vengeance,
   without end, without limit