The Collaborative International Dictionary of English v.0.48:

Incontrovertible \In*con`tro*ver"ti*ble\, a.
   Not controvertible; too clear or certain to admit of dispute;
   indisputable. --Sir T. Browne. --
   In*con`tro*ver"ti*ble*ness, n. -- In*con`tro*ver"ti*bly,
   adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incontrovertibly
    adv 1: in an obvious and provable manner; "his documentary
           sources are demonstrably wrong" [syn: demonstrably,
           provably, incontrovertibly]