The Collaborative International Dictionary of English v.0.48:

Extremely \Ex*treme"ly\, adv.
   In an extreme manner or state; in the utmost degree; to the
   utmost point; exceedingly; as, extremely hot or cold.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extremely
    adv 1: to a high degree or extent; favorably or with much
           respect; "highly successful"; "He spoke highly of her";
           "does not think highly of his writing"; "extremely
           interesting" [syn: highly, extremely]
    2: to an extreme degree; "extremely cold"; "extremely
       unpleasant" [syn: extremely, exceedingly, super,
       passing]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "extremely":
   a bit, a little, a outrance, absolutely, all out,
   beyond all bounds, beyond compare, beyond comparison,
   beyond measure, bloody, completely, damned, darned, dead,
   downright, essentially, ever, exceedingly, exceptionally,
   excessively, extraordinarily, fairly, flat out, fundamentally,
   hellishly, hugely, immeasurably, immensely, in a measure, in a way,
   in some measure, in the extreme, incalculably, indefinitely,
   infinitely, inordinately, kind of, most, notably, outrageously,
   over, overfull, overly, overmuch, parlous, perfectly, pretty,
   purely, quite, radically, rarely, rather, remarkably, scarcely,
   slightly, somewhat, sort of, strikingly, surpassingly, to a degree,
   to some extent, too, totally, uncommon, uncommonly,
   unconditionally, unduly, unequivocally, unusually, utterly, very,
   vitally, with a vengeance