Source : Webster's Revised Unabridged Dictionary (1913)

Illustriously \Il*lus"tri*ous*ly\, adv.
   In a illustrious manner; conspicuously; eminently; famously.
   --Milton.

Source : WordNet®

illustriously
     adv : in an illustrious manner; "Einstein, the illustriously
           famous physicist of the 20th century"